But-For Causation and Chain-of-Distribution Liability in Multi-Drug Overdose Cases: Commentary on United States v. Leslie Brock (6th Cir. 2025)

I. Introduction

The Sixth Circuit’s unpublished decision in United States v. Leslie Brock, No. 25‑5152 (Dec. 23, 2025), adds another detailed application of the Supreme Court’s causation standard in Burrage v. United States to the recurring problem of multi-drug overdose deaths in federal narcotics prosecutions.

Although designated “NOT RECOMMENDED FOR PUBLICATION” and thus non‑precedential within the Sixth Circuit, the opinion is instructive on two central questions under 21 U.S.C. § 841(b)(1)(C)’s “death results” enhancement:

1. How can the government prove “but‑for” causation where the decedent used more than one drug, and the death certificate lists multiple causes of death?
2. How far up the distribution chain can liability extend—specifically, can an upstream supplier be held responsible when an intermediary directly furnishes the fatal dose to the victim?

The panel, in an opinion by Judge Davis, affirms Brock’s conviction for conspiracy to distribute fentanyl where “death resulted” from its use. The court holds that the combination of toxicology evidence, death‑scene circumstances, and circumstantial proof tying the fentanyl to Brock was sufficient for a rational jury to find both:

• that fentanyl was a but‑for cause of the victim’s death, notwithstanding concurrent cocaine use; and
• that the fentanyl used by the victim came from Brock through her sale to an intermediary, Brendan Miller.

In doing so, the court applies and synthesizes a line of Sixth Circuit decisions after Burrage, including Volkman, Hamm, Sadler, Williams, Simer, and Ewing, refining how prosecutors can prove causation and distribution in multi‑drug overdose cases.

II. Factual and Procedural Background

A. Factual Overview

The case arises from the overdose death of N.C., a veterinary student who had returned to Auburn, Alabama, while experiencing fentanyl withdrawal. Lacking a local supplier, she turned to a long‑time friend in Kentucky, Brendan Miller, and asked him to send her fentanyl.

On August 20, 2023:

• Miller used a combination of his own money and funds N.C. sent via CashApp to purchase one gram of fentanyl from defendant Leslie Brock.
• Miller separated approximately 0.3 grams, placed it in a small baggie, taped the baggie inside a greeting card, and mailed it to N.C. on August 21.
• The card contained darkly ironic messages: “Here you go, b[*]tch. Don’t die” and “should have sent Narcan.”

On August 24, N.C. and Miller had a FaceTime call during which:

• N.C. stated she had used the fentanyl Miller sent.
• Miller, who testified he had extensive exposure to fentanyl users, observed her as “very intoxicated,” with slurred speech, heavy eyelids, and general lagging responsiveness.

On August 25, N.C. was found dead in her apartment. Officers observed:

• Fluid aspirated from her nose or mouth that had crystallized—a sign consistent with respiratory failure and prolonged post‑mortem interval.
• Blood pooling (livor mortis) in her legs, indicating she had been deceased for some time.
• On a breakfast bar, in the open and near the body, the greeting card containing the taped baggie with approximately 0.1 grams of fentanyl remaining, and two straws.
• Elsewhere, a powder vial in a backpack and a bag of powder in her bedroom, suspected to be cocaine (though never laboratory‑tested).

Toxicology results showed:

• Fentanyl in N.C.’s blood and oral cavity at a concentration of 15 nanograms per milliliter (ng/mL),
• Cocaine and benzoylecgonine (a metabolite of cocaine), and
• No fentanyl metabolite, which the expert viewed as consistent with recent fentanyl ingestion near the time of death.

The death certificate listed the cause of death as intoxication by both cocaine and fentanyl.

After learning of N.C.’s death, Miller informed Brock that N.C. had died from the fentanyl she had sold him. Despite that notice, Brock continued selling fentanyl to Miller, enabling law enforcement to set up two controlled purchases on October 4 and October 6. A subsequent search of Brock’s residence yielded approximately 20 grams of a fentanyl analogue and 27.48 grams of actual methamphetamine.

B. Procedural History

A federal grand jury indicted Brock on a single count of conspiring with Miller and others to distribute 400 grams or more of a mixture or substance containing fentanyl, with the “death results” enhancement under 21 U.S.C. §§ 841(a)(1), 841(b)(1)(C), and 846.

At trial, the government’s evidence on causation centered on two expert witnesses:

• Hannah Strickland, a forensic scientist, testified to the toxicology findings: the presence of cocaine, benzoylecgonine, and fentanyl in blood and oral cavity.
• Mike Ward, a retired toxicologist, explained:
  • Fentanyl’s mechanism of action—depressing respiration and causing suffocation;
  • The therapeutic range (1–3 ng/mL) versus the 15 ng/mL found in N.C. (≥5 times therapeutic);
  • That such a level is consistent with causing death and could be lethal;
  • That if the fentanyl at that level were absent, N.C. “could very likely have survived”; and
  • That the absence of fentanyl metabolites suggests recent use close to the time of death.

The government also introduced the death certificate documenting combined cocaine and fentanyl intoxication.

At the close of the government’s case, Brock moved for a judgment of acquittal under Rule 29. The district court denied the motion, and the defense presented no evidence. The jury found Brock guilty as charged and made a specific finding that:

N.C. “would not have died but for the use of the same mixture or substance containing a detectable amount of fentanyl distributed by [Brock].”

The district court sentenced Brock to 250 months’ imprisonment and five years of supervised release. On appeal, Brock challenged only the sufficiency of the evidence supporting the death‑results enhancement, not the existence of the conspiracy or her fentanyl distribution generally.

III. Summary of the Sixth Circuit’s Decision

The Sixth Circuit affirmed Brock’s conviction and the application of the § 841(b)(1)(C) enhancement. The key holdings are:

1. Causation: There was sufficient evidence for a rational jury to conclude that fentanyl was a but‑for cause of N.C.’s death, even though the death certificate also listed cocaine and the toxicology showed multiple substances.
2. Distribution / Chain of Distribution: There was sufficient evidence to find that the fentanyl that killed N.C. came from Brock, despite her never directly supplying N.C., because Brock sold the fentanyl to Miller and was thereby within the “chain of distribution” that led to N.C.’s fatal use.

The court rejected Brock’s two central arguments:

• That the presence of cocaine on the death certificate, and Ward’s acknowledgement that cocaine “can contribute” to death, rendered the fentanyl causation evidence insufficient under Burrage; and
• That an “unexplained gap” in the evidence—namely, the presence of cocaine in N.C.’s system and suspected cocaine in her apartment, without proof that Brock supplied cocaine—broke the causal and distribution chain, similar to the defect found in United States v. Ewing, 749 F. App’x 317 (6th Cir. 2018).

Viewing all evidence in the light most favorable to the prosecution and deferring to the jury’s credibility and fact‑finding, the panel held that Brock had not met the “very heavy burden” required to overturn a jury’s verdict on sufficiency grounds.

IV. Detailed Analysis

A. Standard of Review and Framework for Sufficiency Challenges

The opinion begins its legal analysis by restating the basic sufficiency‑of‑the‑evidence standard:

• De novo review of the legal sufficiency question (United States v. Ray, 803 F.3d 244, 262 (6th Cir. 2015)).
• The court asks whether “any rational trier of fact could have found the essential elements of the crime beyond a reasonable doubt” (United States v. Pritchett, 749 F.3d 417, 431 (6th Cir. 2014), quoting Jackson v. Virginia, 443 U.S. 307, 319 (1979)).
• The defendant bears a “very heavy burden” in sufficiency challenges (United States v. Emmons, 8 F.4th 454, 478 (6th Cir. 2021); United States v. Abboud, 438 F.3d 554, 589 (6th Cir. 2006)).
• The court must:
  • View the evidence in the light most favorable to the prosecution (Pritchett);
  • Draw all reasonable inferences in favor of the verdict; and
  • Refrain from reweighing evidence or reassessing witness credibility (United States v. Jackson, 470 F.3d 299, 309 (6th Cir. 2006)).

The court further notes, via Buetenmiller v. Macomb County Jail, 53 F.4th 939, 946 (6th Cir. 2022), that arguments “adverted to in a perfunctory manner” without developed briefing are forfeited. This is why the panel confines its analysis to the death‑results enhancement and does not revisit the underlying conspiracy conviction.

B. Statutory Framework: §§ 841(a)(1), 841(b)(1)(C), and 846

The relevant statutory structure is as follows:

• 21 U.S.C. § 841(a)(1) makes it unlawful to “knowingly or intentionally … distribute, or dispense, or possess with intent to distribute … a controlled substance.”
• 21 U.S.C. § 841(b)(1)(C) sets baseline penalties for certain controlled substances, then provides an enhancement when “death or serious bodily injury results from the use of such substance.”
• 21 U.S.C. § 846 provides that anyone who “conspires” to commit a § 841 offense “shall be subject to the same penalties as those prescribed for the offense.”

Under Burrage v. United States, 571 U.S. 204 (2014), the government must prove two elements for the death‑results enhancement:

1. The defendant knowingly or intentionally distributed a controlled substance; and
2. “Death resulted” from the use of that distributed substance.

Critically, Burrage held that, absent a special “independently sufficient cause” scenario, “results from” requires but‑for causation—that is, the victim would not have died “but for” the use of the defendant’s drug.

C. Causation in Multi-Drug Overdose Cases: Application of Burrage and Sixth Circuit Precedent

1. The Burrage standard and its elaboration in the Sixth Circuit

The Brock panel reiterates that the drug must be:

• either an independently sufficient cause of death, or
• a but‑for cause (i.e., one without which the death would not have occurred),

citing Burrage, 571 U.S. at 218–19 and United States v. Allen, 716 F. App’x 447, 450 (6th Cir. 2017).

The opinion quotes United States v. Volkman, 797 F.3d 377, 392 (6th Cir. 2015), which, in turn, draws from Burrage: but‑for causation exists where the controlled substance “combines with other factors to produce” death, and death would not have occurred “without the incremental effect” of that drug. In other words, concurrent causes do not defeat liability as long as the defendant’s drug is a necessary part of the lethal combination.

2. The role of the death certificate and multi-substance listing

Brock argued that causation was not proved because the death certificate listed both cocaine and fentanyl as contributing causes of death. The court rejects the idea that listing multiple substances precludes a finding that fentanyl was a but‑for cause, relying on:

• United States v. Hixon, 838 F. App’x 961, 964 (6th Cir. 2020), where the death certificate described “acute combined drug toxicity due to cocaine, fentanyl, and gabapentin,” yet the Sixth Circuit still upheld a finding that fentanyl was a but‑for cause based on expert testimony (also provided by toxicologist Mike Ward).

In both Hixon and Brock, the court emphasizes that a death certificate is one piece of evidence, but not dispositive. The key is whether expert testimony and other evidence permit a rational jury to infer that the defendant’s drug was a necessary causal factor in the death.

3. Expert testimony and the “incremental effect” of fentanyl

The panel places significant weight on the toxicological and medical opinion evidence:

• Ward explained that fentanyl depresses breathing, effectively causing suffocation, which matches the physical observation of aspirated fluid crystallizing around N.C.’s nose or mouth.
• The fentanyl concentration (15 ng/mL) far exceeded the therapeutic range (1–3 ng/mL), making death “a fairly serious consequence” at that level.
• Ward opined that, absent this level of fentanyl, N.C. “could very likely have survived.”
• While he acknowledged that cocaine “can contribute” to death, he remained clear that “the reason that [N.C.] no longer lived was” the effects of the fentanyl at that concentration.
• Strickland’s testimony that cocaine had metabolized (evidenced by benzoylecgonine) while fentanyl had not (no fentanyl metabolite was detected) supported a timeline in which cocaine was taken earlier, and fentanyl was used close in time to death.

This combination of:

• a very high fentanyl level capable of causing death;
• temporal proximity between fentanyl use and death (FaceTime call the evening before, no metabolite); and
• expert opinion that, absent fentanyl, she likely would have survived

is exactly the type of evidence that satisfies Burrage’s “incremental effect” idea: even with cocaine present, the jury could rationally find that fentanyl was a necessary causal factor.

The decision thus joins prior Sixth Circuit cases emphasizing strong toxicological proof of the individual drug’s contribution:

• United States v. Hamm, 952 F.3d 728, 738 (6th Cir. 2020): medical testimony that, “had [the victim] not had that level of carfentanil, he could have survived,” sufficed for but‑for causation.
• United States v. Allen, 716 F. App’x 447, 451 (6th Cir. 2017): fentanyl in victim’s blood was lethal “to a medical certainty,” supporting the enhancement.
• United States v. Smith, 656 F. App’x 70, 74 (6th Cir. 2016): testimony that the victim “would not have died had she not consumed oxycodone” satisfied Burrage.

4. Rejecting the “more certainty is required” argument

Brock argued that Ward’s testimony was not sufficiently definitive and that the government needed more expert evidence, especially given cocaine’s presence. The court answers this in several ways:

• Sufficiency review examines whether a reasonable jury could find the elements beyond a reasonable doubt, not whether more evidence might have made the case stronger.
• Ward’s core causal statement—that fentanyl was “the reason that [N.C.] no longer lived”—was explicitly clear, even if he acknowledged other drugs might have contributed.
• Strickland’s metabolite testimony corroborated Ward’s explanation of temporal sequencing, bolstering the inference that fentanyl’s “marginal effect” turned a non‑fatal situation into a fatal one.

Additionally, the court notes that the government is not required to “remove every reasonable hypothesis except that of guilt” or to exclude every other theoretical cause, citing:

• United States v. Sadler, 24 F.4th 515, 539 (6th Cir. 2022) (quoting United States v. LaVictor, 848 F.3d 428, 456 (6th Cir. 2017)).

Sadler also emphasized that a victim’s response to Narcan showed opioid intoxication as the cause, another example of how specific medical facts can point to an opioid as the decisive factor among multiple drugs. In Brock, the high fentanyl level and lack of metabolite play a similar clarifying role.

5. The court’s choice not to decide “independent sufficiency”

Having found the evidence ample to support but‑for causation, the court explicitly declines to decide whether fentanyl was also an “independently sufficient” cause of death. This is consistent with Burrage and Allen: once but‑for causation is satisfied, the enhancement stands, and there is no need to consider whether fentanyl alone, standing apart from cocaine, would have been sufficient to produce death.

D. Distribution and Chain-of-Distribution Liability

1. Legal framework: downstream deaths and upstream suppliers

The panel relies on a line of Sixth Circuit cases establishing that:

• § 841(b)(1)(C) applies not only to those who directly hand drugs to the victim but to anyone within the “chain of distribution” that led to the fatal dose.
• United States v. Williams, 998 F.3d 716, 734 (6th Cir. 2021), is particularly important: it held that the enhancement can reach “others who were in the chain of distribution that caused the victim’s death or injury.”
• United States v. Davis, 970 F.3d 650, 656 (6th Cir. 2020), held that the government need not show a direct, personal connection between the defendant and the victim if the drugs the defendant distributed are the same drugs that caused death.

Thus, the question is not whether Brock directly supplied cocaine (she did not), nor whether she personally mailed anything to N.C. (she did not), but whether:

1. She knowingly distributed fentanyl (conceded); and
2. The fentanyl she distributed formed part of the causal chain that ended in N.C.’s death.

2. Evidence tying Brock’s fentanyl to N.C.’s fatal use

The court underscores several circumstantial facts allowing the jury to infer that the fentanyl N.C. ingested was the very fentanyl Brock sold to Miller:

• No local fentanyl supplier: N.C. was in withdrawal and sought fentanyl specifically from Miller in Kentucky, suggesting this was her source for fentanyl.
• Financial and temporal chain:
  • N.C. sent Miller $30 on August 14 to buy fentanyl for her.
  • Miller purchased from Brock on August 20 and mailed the fentanyl on August 21.
  • N.C. reported using “the fentanyl that Miller sent” on August 24’s FaceTime call.
• Physical evidence at the scene: The greeting card with the taped fentanyl baggie was on the breakfast bar near N.C.’s body, with two straws, and some of the same baggie’s contents (approximately 0.1 grams) remained.
• Toxicology match: Fentanyl was found in N.C.’s blood and oral cavity, even though the only overtly identified fentanyl in the apartment was in the card from Miller (whether any fentanyl analogue from Brock’s later-seized stash matches is not discussed but is unnecessary to the sufficiency analysis).
• Tight temporal proximity: Only five days elapsed from Brock’s sale (August 20) to N.C.’s death (August 25). The FaceTime call and immediate aftermath created a narrow window in which the fentanyl was both used and lethal (see United States v. Simer, 835 F. App’x 60, 65 (6th Cir. 2020) (noting “tight temporal proximity” as supporting causation)).

Taken together, these facts permitted a rational juror to conclude that:

• the fentanyl that Brock sold to Miller was the fentanyl Miller mailed,
• this was the fentanyl N.C. ingested, and
• that ingestion was a but‑for cause of her death.

This satisfied the requirement that Brock was in the causal chain of distribution leading to the fatal dose, as in Williams.

3. Distinguishing United States v. Ewing

Brock relied on United States v. Ewing, 749 F. App’x 317 (6th Cir. 2018), which vacated a death‑results enhancement because of an “unexplained gap in the evidence.” In Ewing:

• The government charged the defendant with distributing a mixture of heroin and fentanyl that allegedly caused death.
• The victim’s blood showed no heroin or heroin metabolites—only fentanyl and other drugs—while heroin appeared only in urine.
• Because the government offered no explanation for the absence of heroin in the blood (where lethal drugs ordinarily appear), the court found causation unproven as to the specific “mixture” charged.

The Brock panel finds Ewing inapposite for several reasons:

• No missing charged drug: In Ewing, the charged mixture included heroin, but heroin did not appear in the victim’s blood—creating a mismatch between the indictment and the proof. Here, the charged offense is distribution of fentanyl, and fentanyl did appear in N.C.’s blood and oral cavity.
• No unexplained toxicological gap: In Ewing, the absence of heroin in blood created a scientific puzzle the government never solved. In Brock, by contrast, the evidence cohered: fentanyl was present at lethal levels; its absence as a metabolite supported recent use; and N.C.’s statements and the physical evidence explained where it came from.
• Cocaine’s role differs: Cocaine’s presence in N.C.’s system and suspected cocaine in her apartment do not undermine the proof that fentanyl came from Brock. Unlike the missing heroin in Ewing, the “extra” cocaine here doesn’t contradict the core theory; it merely presents an alternative or contributory factor, which the jury could reasonably discount as the primary cause in light of the strong fentanyl evidence.

The court also references United States v. Assfy, No. 20‑1630, 2021 WL 2935359, at *6 (6th Cir. July 13, 2021) (unpublished), which rejected an Ewing‑based argument where there was no evidence that the defendant sold a drug that did not appear in the victim’s blood. Brock falls into the same category: there is no such mismatch.

In short, what was fatal to the government’s case in Ewing—a charged drug absent from the key forensic evidence—is absent here. The court characterizes the cocaine‑based arguments as “simply speculative possibilities already rejected by the jury” (quoting Simer, 835 F. App’x at 66), which an appellate court will not re‑weigh on sufficiency review.

E. Scientific and Forensic Evidence: How It Is Used

The opinion illustrates several recurring evidentiary themes in overdose prosecutions:

• Drug levels versus therapeutic range: Showing that a drug concentration (15 ng/mL fentanyl) massively exceeds therapeutic levels helps move from “possible cause” to “likely cause.”
• Metabolite analysis for timing: The presence of a cocaine metabolite but absence of a fentanyl metabolite supports a sequence of earlier cocaine use and very recent fentanyl use—bridging the gap between ingestion and death.
• Scene evidence tied to specific drugs: The card, baggie, and straws found near N.C.’s body support the inference that the fentanyl from that baggie—the one sent by Miller—was the substance ingested close to death.
• Physical signs of overdose: Aspirated fluid from the nose/mouth, consistent with respiratory depression and suffocation, aligns specifically with fentanyl’s pharmacology as described by Ward.

The court’s treatment of this evidence underscores that “scientific certainty” is not required; rather, the combination of expert explanation and circumstantial facts can permit a reasonable jury to find but‑for causation beyond a reasonable doubt.

F. Narrowing the Issues on Appeal and Forfeiture

A brief but important doctrinal point is the court’s note that Brock attacked only the death‑results enhancement on appeal. By citing Buetenmiller and the rule against undeveloped arguments, the court:

• Limits its review to the narrow questions of causation and distribution of the death‑causing fentanyl;
• Preserves the conspiracy finding and the basic § 841 liability as conceded or forfeited; and
• Illustrates the importance of careful appellate issue‑selection and briefing.

G. Relationship to and Refinement of Sixth Circuit Precedent

United States v. Brock does not announce new law, but it harmonizes and consolidates several strands of existing precedent:

• Burrage and “incremental effect” causation: Following Volkman, Hamm, Allen, and Smith, Brock confirms that strong medical testimony linking the defendant’s drug to the victim’s death, even amid other drugs, suffices for but‑for causation.
• Death certificate not dispositive: Hixon and Brock both treat multi‑drug death certificates as non‑determinative where expert testimony isolates the defendant’s drug as the decisive factor.
• Chain-of-distribution liability: Building on Williams and Davis, Brock illustrates that upstream suppliers who transfer drugs to intermediaries can face the death‑results enhancement when the intermediate distribution leads to a fatality.
• Limit of Ewing: Brock narrows Ewing to its facts: a case where the government failed to reconcile a charged drug’s absence from the core forensic evidence. It clarifies that Ewing does not require prosecutors to negate every hypothetical cause or to prove exclusive causation by the charged drug, as long as the charged drug is present and demonstrably necessary to the death.

V. Complex Concepts Simplified

1. “But-For” Causation

“But‑for” causation asks a simple counterfactual question:

Would the victim have died if the defendant’s drug had not been used?

If the answer is “no”—i.e., the death would not have occurred without that drug—then the drug is a but‑for cause. This remains true even if other drugs also contributed, as long as the defendant’s drug is a necessary part of the lethal combination.

2. “Independently Sufficient” Cause

In theory, a cause is “independently sufficient” if it alone would have been sufficient to cause the death, even without other contributing factors. Burrage suggests that when a defendant’s drug independently suffices to cause death, causation can be established under certain conditions even in a multi‑cause context. In practice, the Sixth Circuit typically focuses on the but‑for test, as it did in Brock.

3. Death-Results Enhancement under § 841(b)(1)(C)

This enhancement does not create a separate crime but increases the penalty for a § 841 offense when:

1. The defendant distributes or conspires to distribute a controlled substance, and
2. “Death or serious bodily injury results from the use of such substance.”

In drug‑death prosecutions, this enhancement often drives the sentencing exposure (including potential mandatory minimums and higher statutory maxima), making the causation element a focal battleground at trial.

4. Chain-of-Distribution Liability

“Chain of distribution” liability means that an upstream distributor (like Brock) can be held responsible for a death that occurs downstream (to N.C.) so long as:

• The drug causing death is traceable to the defendant’s distribution; and
• The government can prove causation and the elements of the underlying § 841 violation.

The defendant need not know the victim or personally transfer the drugs to the person who dies.

5. Metabolites and Timing of Drug Use

A metabolite is a substance formed when the body chemically breaks down a drug. Forensic toxicologists use metabolites to:

• Confirm which drugs were used; and
• Estimate how long before death the drug was taken (recent use often shows high parent drug and low/no metabolite; older use shows more metabolite).

In Brock, the presence of a cocaine metabolite and absence of a fentanyl metabolite supported an inference that cocaine use preceded fentanyl use, and that fentanyl was ingested close to the time of death.

6. “Therapeutic Range” versus Lethal Levels

The therapeutic range is the concentration range of a drug considered safe and effective for intended medical use. For fentanyl, 1–3 ng/mL is therapeutic. Concentrations much higher (e.g., 15 ng/mL in N.C.’s blood) significantly raise the risk of overdose and death. Courts often rely on medical experts to interpret these figures.

VI. Likely Impact and Future Litigation

A. Effect on Multi-Drug Overdose Prosecutions

Although unpublished, Brock is likely to be cited in briefs and district court rulings as persuasive authority in several ways:

• Reaffirmation that multiple drugs on a death certificate do not defeat causation: Prosecutors can proceed with death‑results enhancements even where coroners list “combined drug intoxication,” as long as they present convincing expert testimony and evidence that the defendant’s drug was a but‑for cause.
• Importance of toxicology detail: The opinion underscores the value of precise concentration data, therapeutic ranges, and metabolite analysis in convincing a jury and sustaining a verdict on appeal.
• No requirement to negate all other drugs: Defense arguments that another drug (e.g., cocaine) “could” have caused death will not prevail in sufficiency challenges if the government presents adequate evidence that the charged drug’s “incremental effect” was necessary to the fatal outcome.

B. Guidance for Prosecutors and Defense Counsel

For prosecutors:

• Obtain and present detailed expert testimony linking concentration levels, metabolite patterns, and specific pharmacological effects to the manner of death.
• Develop clear, narrative evidence of the distribution chain—from upstream supplier to intermediary to victim—especially through communications, money trails (e.g., CashApp), and physical evidence like packaging.
• Anticipate defense reliance on Ewing by ensuring there is no mismatch between the charged drug(s) and what appears in toxicology reports.

For defense counsel:

• Focus on creating a credible alternative causation theory with expert testimony, rather than relying solely on the existence of other drugs in the system.
• Scrutinize how closely the toxicology results match the government’s theory—where there are gaps or inconsistencies, Ewing still provides a potent tool.
• Consider challenging the reliability or interpretation of metabolite evidence, especially where timing of use is central to the government’s theory.

C. Chain-of-Distribution and Conspiracy Liability

Brock reinforces that conspirators and upstream suppliers remain exposed to the harsh penalties associated with § 841(b)(1)(C) even when an intermediary directly handles the fatal distribution. In the fentanyl era, this confirms a broad potential reach of federal criminal liability within drug networks for overdose deaths.

D. Limits of the Opinion

Being unpublished, Brock is not binding precedent, but it is consistent with—and therefore reinforces— the trajectory of Sixth Circuit authority post‑Burrage. It offers little support to defendants seeking a more demanding causation standard beyond but‑for causation as elaborated in Volkman and progeny.

VII. Conclusion

United States v. Brock is a fact‑intensive but doctrinally important application of the Burrage causation framework in a modern fentanyl case involving multiple substances. The Sixth Circuit holds that:

• A death certificate that lists multiple drugs as causes does not preclude a finding that one of them—here, fentanyl—is a but‑for cause of death, so long as expert testimony and forensic evidence support that conclusion.
• Upstream suppliers in a drug distribution chain can face the death‑results enhancement where their product reaches a victim through an intermediary and is a but‑for cause of the victim’s death.
• Speculative alternative‑cause scenarios (such as untested cocaine in the apartment) do not undermine a verdict where the government’s evidence reasonably ties the charged substance to both causation and distribution.

In the broader legal context, Brock consolidates a now‑familiar message from the Supreme Court and the Sixth Circuit: overdose deaths in multi‑drug scenarios will be assessed through the lens of but‑for causation, informed by rigorous toxicological and circumstantial evidence, and liability can extend across the entire chain of distribution. Although unpublished, Brock is a significant and instructive addition to the body of case law governing drug‑related death enhancements under § 841(b)(1)(C).