United States v. Palmer: Narrowing Rule 704(b) After Diaz and Rejecting Padilla-Based Collateral Attacks on State Pleas

I. Introduction

In United States v. Gregory Maxwell Palmer, No. 23‑4538 (4th Cir. Nov. 18, 2025), the Fourth Circuit addressed three significant questions at the intersection of criminal procedure, evidence, and immigration-related criminal liability:

When does preindictment delay in charging federal naturalization fraud violate due process?
Can a defendant collaterally attack a prior state guilty plea in federal court on the ground that his lawyer failed to advise him of immigration consequences under Padilla v. Kentucky?
How narrowly must courts read Federal Rule of Evidence 704(b) after the Supreme Court’s decision in Diaz v. United States when excluding expert opinion on a defendant’s mental state?

The panel (Chief Judge Diaz, joined by Judges Wilkinson and Wynn) affirmed Palmer’s conviction for naturalization fraud under 18 U.S.C. § 1425(a), upholding the indictment and the use of his prior state conviction, while finding—but ultimately deeming harmless—a Rule 704(b) error in excluding defense expert testimony.

The decision is especially important for two reasons:

It is an early published application in the Fourth Circuit of the Supreme Court’s 2024 decision in Diaz v. United States, 602 U.S. 526 (2024), sharply narrowing the scope of Rule 704(b) and clarifying when experts may testify on issues closely related to a defendant’s mental state.
It firmly rejects efforts to carve out a special exception for Padilla-type ineffective-assistance errors from the longstanding rule that prior state convictions generally cannot be collaterally attacked in subsequent federal proceedings.

II. Factual and Procedural Background

A. Naturalization and the Undisclosed Conduct

In 2011, Gregory Palmer applied for U.S. citizenship. The naturalization application (Form N‑400) included Question 15:

“Have you ever committed a crime or offense for which you were not arrested?”

Palmer answered “No” on the written form. At his in‑person naturalization interview, a USCIS officer:

Administered English reading, writing, and civics tests, which Palmer passed; and
Went through each question on the application orally, including Question 15, to which Palmer again answered “No.”

On that basis, USCIS approved the application, and Palmer became a naturalized U.S. citizen.

B. The State Conviction

In 2013, in North Carolina state court, Palmer pled guilty to attempted statutory rape. In doing so, he:

Admitted that in 2008—three years before his naturalization—he had engaged in a sexual act with a 13‑year‑old.
Entered the plea in exchange for the dismissal of four counts of statutory rape and four counts of indecent liberties with a child.
Told the court he understood the proceedings, could read and write at an eleventh-grade level, and was pleading guilty because he was guilty.

The state court sentenced him to 157–198 months in prison.

C. The Federal Naturalization-Fraud Charge

In 2021, just within the ten-year statute of limitations for naturalization fraud, a federal grand jury indicted Palmer under 18 U.S.C. § 1425(a). The government alleged that:

Palmer had “knowingly” provided false information on his naturalization application by denying he had ever committed a crime for which he had not been arrested.
The undisclosed 2008 sexual conduct, admitted in the 2013 plea, made his 2011 “No” answer false.

D. Pretrial Motions

Motion to dismiss for preindictment delay. Palmer argued that the eight-year delay between his alleged misrepresentation (2011) and his federal indictment (2021) violated due process. He claimed prejudice because his mother, who died during this period, was allegedly a key witness who would have testified about his lifelong cognitive and language difficulties and his inability to understand Question 15.
Motion to suppress the state guilty plea and conviction. Palmer contended that his 2013 guilty plea was constitutionally invalid because his state lawyer had failed to advise him of the immigration and naturalization consequences, in violation of Padilla v. Kentucky, 559 U.S. 356 (2010). He sought to suppress the state conviction so it could not be used to prove naturalization fraud.

The district court:

Denied the motion to dismiss, without an evidentiary hearing, for failure to show actual prejudice.
Denied the motion to suppress, holding that Palmer’s attack on the state plea was an impermissible collateral attack on a presumptively valid state conviction.

E. Expert Evidence and Trial

For trial, Palmer retained a speech-language pathologist, Dr. Kathleen Fahey. She had two main opinions:

Palmer had a low IQ and a language‑literacy developmental disorder; he performed in the “poor to very poor” range on language tests and had approximately a fourth-grade reading level.
Based on that, and on the fact that Question 15 had an eighth-grade “readability” level, Palmer could not have understood the question he answered falsely.

Before trial, the district court limited her testimony under Federal Rule of Evidence 704(b):

It excluded her opinion that Palmer did not understand Question 15.
It also excluded her specific “grade‑level” opinions (that Palmer read at a fourth‑grade level while Question 15 was written at an eighth‑grade level), because those opinions were viewed as too closely connected to the issue of his knowledge.

At trial, three witnesses testified:

The immigration officer, who described Palmer’s successful performance on the naturalization tests and his oral confirmation of his answers without signs of confusion.
Palmer’s ex‑wife, who testified that she helped him fill out the application, that he wrote letters to her, and that he read western novels for pleasure.
Dr. Fahey, who testified extensively about his language and literacy deficits, but without the excluded “grade‑level” opinions or a direct conclusion that he did not understand Question 15.

The jury found that Palmer knowingly answered Question 15 falsely and convicted him of naturalization fraud. Under 8 U.S.C. § 1451(e), the district court was required to revoke his citizenship and void his certificate of naturalization. Palmer was sentenced to six months’ imprisonment, concurrent with his state sentence.

III. Summary of the Fourth Circuit’s Decision

The Fourth Circuit affirmed on all grounds, holding:

No unconstitutional preindictment delay. Palmer failed to show “substantial actual prejudice,” particularly given the speculative nature of what his deceased mother would have said and the availability of other sources for similar evidence. Without actual prejudice, the court did not reach the government’s reasons for delay.
No suppression of the state guilty plea based on Padilla error. The court characterized the motion as an impermissible collateral attack on a presumptively valid state conviction. Citing Custis and Coss, it refused to create an exception for Padilla-type ineffective-assistance claims, holding that the only recognized exception for collateral attacks in such contexts is a complete denial of counsel.
Rule 704(b) error, but harmless. Applying the Supreme Court’s recent decision in Diaz v. United States, the court held that Rule 704(b) was read too broadly: Dr. Fahey’s “grade‑level” opinions should not have been excluded because they did not directly opine on Palmer’s mental state. However, because similar evidence about his language deficits was admitted and the case against him was strong, the error was harmless and did not warrant reversal.

IV. Detailed Analysis

A. Preindictment Delay and Due Process

1. The Legal Framework

Statutes of limitations are the primary protection against stale prosecutions. Here, everyone agreed that the indictment was filed within the applicable ten‑year limitations period. But even timely indictments can violate due process in “rare” cases of prejudicial preindictment delay.

The Fourth Circuit follows the two‑step framework described in:

United States v. Lovasco, 431 U.S. 783, 789 (1977) (statutes of limitations are the primary guarantee; due process violations from delay are “limited” and require more than mere passage of time);
United States v. Uribe‑Rios, 558 F.3d 347, 358 (4th Cir. 2009) (delay must violate “fundamental conceptions of justice or the community’s sense of fair play and decency”).

Under this line of cases, as restated in Villa and Shealey, a defendant must:

First, show “substantial actual prejudice” to the defense caused by the delay. See United States v. Villa, 70 F.4th 704, 716 (4th Cir. 2023); United States v. Shealey, 641 F.3d 627, 633–34 (4th Cir. 2011).
Only if that heavy threshold is met does the court then balance the prejudice against the government’s reasons for the delay. Uribe‑Rios, 558 F.3d at 358.

“Substantial actual prejudice” means the defendant was:

“meaningfully impaired in his ability to defend against the state’s charges to such an extent that the disposition of the criminal proceeding was likely affected.” (Shealey, 641 F.3d at 634)

2. Lost-Witness Claims: The Jones v. Angelone Test

When the alleged prejudice is the loss of a witness, the Fourth Circuit looks to Jones v. Angelone, 94 F.3d 900, 907–08 (4th Cir. 1996). The defendant must:

Identify the witness he would have called.
Demonstrate, with specificity, the expected content of the witness’s testimony.
Show that he made serious attempts to locate the witness (if the witness is missing rather than deceased).
Show that the information was not available from other sources.

These are cumulative; failure on any one is fatal, and even satisfying all four does not automatically establish that the prejudice is “substantial.”

3. Application to Palmer: Speculative Testimony and Cumulative Evidence

Palmer’s theory was that his mother, had she lived, would have testified about his:

Longstanding cognitive and language difficulties;
Limited reading ability; and
Inability to understand questions like Question 15.

The court rejected this as insufficiently specific and too speculative. It relied on:

United States v. Automated Medical Laboratories, Inc., 770 F.2d 399, 404 (4th Cir. 1985), where the court held that a defendant’s own description of what a missing witness “would have said,” without corroboration, leaves the supposed testimony “highly speculative.”

Two points were central:

Lack of non-speculative content. Palmer offered only his own assertions about what his mother would have testified to; he produced no prior statements, affidavits, or other corroboration. This failed the second Jones factor.
Cumulative nature of the anticipated testimony. Even taking his description at face value, the same or similar information was available elsewhere:
- Dr. Fahey, who described his language and literacy deficits in detail.
- His ex‑wife, who described his difficulties and how she helped with the application.
- Other potential witnesses (siblings, teachers, employers, friends) who could have spoken to his cognitive and language struggles.
This undermined the fourth Jones factor—uniqueness of the lost testimony.

In light of the robust evidence actually presented and other potential sources of similar testimony, the court concluded that the absence of Palmer’s mother did not likely affect the outcome. Hence, there was no “substantial actual prejudice.”

4. No Need for an Evidentiary Hearing

Palmer also argued that the district court erred by denying his motion without an evidentiary hearing. The Fourth Circuit cited United States v. Bowman, 106 F.4th 293, 300 (4th Cir. 2024), which requires a hearing only if the motion raises a “material factual dispute.”

Because Palmer’s proffer about his mother’s anticipated testimony was speculative and conclusory, there was no material factual dispute that required live testimony. Thus, denying a hearing was not an abuse of discretion.

5. Impact of the Preindictment-Delay Ruling

The ruling reinforces the Fourth Circuit’s stringent approach to due process challenges based on preindictment delay:

Being within the statute of limitations is not dispositive, but it puts a heavy thumb on the scale in favor of the government;
Defendants must present concrete, non-speculative evidence of what a lost witness would say and why it would likely alter the outcome;
Loss of cumulative or replaceable testimony does not constitute “substantial actual prejudice,” even when the delay is long and the government offers weak or no justification.

For practitioners, the case underscores the importance of:

Documenting anticipated testimony from vulnerable witnesses (e.g., elderly or ill family members) early;
Identifying and preserving alternative sources of similar evidence; and
Recognizing that, in the Fourth Circuit, speculative claims about what a deceased witness “would have said” will not support a due process claim for delay.

B. Collateral Attacks on State Pleas and Padilla Errors

1. The Presumption of Validity for Prior Convictions

The district court treated Palmer’s motion to suppress the state guilty plea and conviction as a collateral attack on that state judgment. The Fourth Circuit agreed.

Under longstanding doctrine:

State convictions are “presumptively valid” when used in subsequent proceedings. See United States v. Locke, 932 F.3d 196, 199 (4th Cir. 2019); Lackawanna Cnty. Dist. Att’y v. Coss, 532 U.S. 394, 403–04 (2001).
Generally, a defendant may not challenge a prior conviction—or use a later forum to invalidate it—except in narrow circumstances.

2. The Gideon-Only Exception and Custis

The Supreme Court has recognized only a single exception in the context of using prior convictions to enhance or support new federal proceedings:

When the prior conviction was obtained in violation of the right to counsel, i.e., where counsel was entirely denied. See Gideon v. Wainwright, 372 U.S. 335 (1963); Burgett v. Texas, 389 U.S. 109, 115 (1967).

In Custis v. United States, 511 U.S. 485 (1994), the Supreme Court held that at federal sentencing, a defendant normally cannot attack prior state convictions used to enhance his sentence, except in the narrow “Gideon” context:

“[W]e think that since the decision in Burgett, our cases have allowed defendants to challenge only the validity of prior convictions obtained in violation of the right to counsel.” (Custis, 511 U.S. at 496)

The Court explicitly refused to extend this exception to other types of ineffective-assistance claims.

3. Palmer’s Padilla-Based Theory

Palmer argued that his 2013 plea was constitutionally defective because his lawyer failed to advise him that pleading guilty could result in:

Immigration consequences; and in particular,
Potential exposure to denaturalization or naturalization fraud charges.

That is a classic Padilla claim. In Padilla v. Kentucky, 559 U.S. 356 (2010), the Supreme Court held that:

Defense counsel has a Sixth Amendment duty to advise a noncitizen defendant of clear deportation risks of a guilty plea; and
Failure to do so constitutes deficient performance under Strickland v. Washington, 466 U.S. 668 (1984).

Palmer urged the Fourth Circuit to recognize a new, limited exception that would allow collateral attacks on prior convictions in federal proceedings when the prior plea contained a Padilla-type error.

4. The Fourth Circuit’s Response: No Special Padilla Exception

The court rejected Palmer’s argument, aligning itself with its own unpublished ruling in United States v. Vongphakdy, No. 22‑4593, 2023 WL 6638122, at *2 (4th Cir. Oct. 12, 2023), and with the reasoning of Custis and Coss.

The key points:

Padilla errors are a species of ineffective assistance. They are serious but do not amount to “jurisdictional defects” like the total absence of counsel.
No principled way to distinguish Padilla from other IAC claims. The court explicitly held that it “find[s] no principled basis to distinguish Padilla errors from other ineffective assistance of counsel claims.”
Padilla does not expand the Custis exception. While Padilla imposes duties on counsel and provides grounds for challenging a conviction in a proper forum (e.g., state post-conviction or habeas), it does not authorize collateral attacks whenever the conviction is later used in federal court.

Thus, unless Palmer could show he was entirely denied counsel at his state plea—a claim he did not make—his state conviction remained presumptively valid and usable as evidence in his federal naturalization-fraud prosecution.

5. Practical Impact

The ruling has significant implications for noncitizen defendants who later face federal charges tied to prior state convictions:

Padilla claims must be raised in the original jurisdiction (state post-conviction, direct appeal, habeas), not in a later federal prosecution where the conviction is being used.
The only recognized exception for collateral attacks remains Gideon-type total denial of counsel. Ineffective assistance, including failure to advise on immigration consequences, is not enough.
For naturalized citizens, the stakes are high. A prior conviction that is later used to establish naturalization fraud can, as here, lead to automatic loss of citizenship under 8 U.S.C. § 1451(e), yet it remains insulated from collateral attack under this line of cases.

C. Rule 704(b), Diaz v. United States, and Expert Testimony on Mental State

1. The Text and Purpose of Rule 704

Federal Rule of Evidence 704 has two parts:

Rule 704(a): “An opinion is not objectionable just because it embraces an ultimate issue.”
Rule 704(b): In a criminal case, an expert witness “must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense.”

In other words:

Experts may address the ultimate issue (e.g., causation, reasonableness),
But they cannot testify directly that “the defendant intended X,” “knew Y,” or “lacked the capacity to form intent” when that mental state is an element of the offense or a defense.

2. Diaz v. United States: A Narrow Reading of 704(b)

In Diaz v. United States, 602 U.S. 526 (2024), the Supreme Court clarified the scope of Rule 704(b). The expert in Diaz testified that:

“Most drug couriers know that they are transporting drugs.”

The question was whether that testimony violated Rule 704(b) by effectively telling the jury that the defendant—who was alleged to be a drug courier—must have known she was transporting drugs.

The Supreme Court held that:

Rule 704(b) “targets conclusions about whether a certain fact is true: the defendant did or did not have a mental state or condition.”
Testimony about the mental state of a group or “most couriers” is not testimony directly about this defendant’s mental state.
So long as the expert leaves room for the jury to make its own inference about the defendant’s state of mind, the testimony does not run afoul of Rule 704(b).

The upshot is a narrow construction of Rule 704(b): it bars only explicit opinions that the defendant did or did not have the requisite state of mind; it does not bar opinions that are merely strongly suggestive or that provide context from which a jury could infer mental state.

3. The District Court’s Exclusion of Dr. Fahey’s Opinions

Dr. Fahey’s proposed testimony had three relevant components:

Palmer has low IQ and a language‑literacy developmental disorder.
Palmer reads at a fourth‑grade level, whereas Question 15 is written at an eighth‑grade readability level.
Therefore, Palmer did not understand Question 15.

The district court excluded (2) and (3) under Rule 704(b). On appeal:

Palmer did not challenge the exclusion of (3)—the direct conclusion that he did not understand Question 15.
He did challenge the exclusion of (2) (the grade-level comparison), arguing that it provided relevant information about his capacity without directly stating that he lacked knowledge.

4. The Fourth Circuit’s Application of Diaz

The Fourth Circuit recognized that Diaz post-dated Palmer’s trial and provided new guidance. Drawing from Diaz, the court held:

“If the expert’s opinion leaves room for the jury to draw its own conclusion about the defendant’s mental state, Rule 704(b) doesn’t block it.”

Applying that standard, the court concluded:

The excluded “grade‑level” opinions did not compel the conclusion that Palmer failed to understand Question 15.
There remained “a little daylight” between stating:
- “Palmer reads at a fourth-grade level; Question 15 has an eighth-grade readability;” and
- “Palmer did not understand Question 15.”
That “daylight,” however small, is enough under Diaz to keep the testimony outside Rule 704(b)’s prohibition.

Thus, the district court erred in excluding Dr. Fahey’s grade‑level opinions under Rule 704(b).

5. Harmless Error Analysis

Finding an evidentiary error does not automatically result in reversal. Under United States v. Hedgepeth, 418 F.3d 411, 419 (4th Cir. 2005), the question is whether the error “could have affected the verdict.”

Several factors led the court to deem the error harmless:

Substantial overlapping evidence was admitted. Dr. Fahey did testify that Palmer:
- Scored in the first and second percentiles on reading tests;
- “Performed in the poor to very poor range” on all language measures;
- Had serious and longstanding language-processing deficits.
Other evidence favored the government’s knowledge theory. The jury heard that:
- Palmer told the state court he read at an eleventh-grade level;
- He passed the naturalization reading, writing, and civics tests;
- He wrote letters and read western novels for pleasure;
- The USCIS officer saw no signs of confusion as Palmer orally confirmed each question, including Question 15.
The excluded “grade‑level” testimony was, in context, cumulative. It would have provided another lens on his reading ability but would not have qualitatively changed the defense theory or the evidentiary picture.

Given this record, the court could not say that admitting the grade‑level opinions would likely have changed the jury’s conclusion that Palmer knowingly misrepresented his criminal history. Accordingly, the error was harmless.

6. Impact on Future Expert Testimony

Palmer is an important, early Fourth Circuit application of Diaz and has several practical consequences:

Narrower use of Rule 704(b) as an exclusionary tool. Courts should not use 704(b) to bar:
- Expert testimony about a defendant’s cognitive abilities, literacy level, or comprehension capacity;
- Opinions about general group characteristics (“most people with X condition struggle to understand Y”);
- Or data points (e.g., test scores, grade-level metrics) that shed light on the defendant’s capabilities,
unless the expert is directly stating that “this defendant did (or did not) know/intend/understand” a specific legal element.
Defense experts may be more robustly utilized. In fraud, perjury, or false-statement cases, experts can discuss mental capacity, literacy, and comprehension to support the argument that the defendant lacked the requisite knowledge, so long as they stop short of saying “he did not know.”
Prosecutors must rely on other rules (e.g., Rules 702, 403) to limit problematic expert testimony. If expert evidence is unreliable, speculative, or unduly confusing, Rule 702 or 403—not 704(b)—is the appropriate ground for exclusion.

V. Simplifying Key Legal Concepts

A. Preindictment Delay vs. Statute of Limitations

Statute of limitations: A fixed legal deadline; if the government files charges after the limitations period expires, the case is usually barred, with no need to show prejudice.
Preindictment delay due process claim: Even when the indictment is timely, a defendant can argue that the government waited so long that:
- Key evidence was lost, and
- His ability to defend himself was significantly harmed,
but he must show concrete, substantial prejudice that likely changed the outcome.

B. Collateral Attack on a Prior Conviction

A collateral attack is when a defendant in one case tries to undermine a conviction from another case, rather than attacking it in its original forum (direct appeal, state post‑conviction, or habeas).

In federal court, when a prior state conviction is used (for sentencing, as an element, or as evidence), the general rule is:

You cannot relitigate the validity of that prior conviction;
Except if you were completely denied a lawyer in the prior case (a Gideon violation);
Ineffective assistance claims (like Padilla errors) must be litigated in the original jurisdiction.

C. Rule 704(b) and Expert Testimony on Mental State

Rule 704(b) prevents experts in criminal cases from telling the jury:

“The defendant intended to defraud,”
“The defendant knew the statement was false,” or
“The defendant lacked the capacity to form intent,” when intent or knowledge is an element.

But, especially after Diaz and Palmer, it does not prevent:

Testimony about what most people in similar circumstances know;
Describing test results, IQ scores, reading levels, or cognitive diagnoses;
Explaining what kinds of documents or questions are generally difficult to understand for people with certain impairments;
So long as the expert leaves the final, specific inference about this defendant’s mental state to the jury.

D. Harmless Error

Not every trial error leads to reversal. An error is “harmless” if the appellate court concludes that:

The error could not reasonably have affected the verdict.

In practice, courts look at:

The strength of the properly admitted evidence;
Whether the excluded or admitted evidence was cumulative;
The centrality of the evidence to the contested issues.

VI. Broader Significance and Conclusion

A. Key Takeaways from United States v. Palmer

Due process challenges to preindictment delay remain very hard to win in the Fourth Circuit.
- Even in a case with long delay and a deceased witness, a defendant must show specific, non-speculative, unique testimony that likely would have changed the outcome.
- Mere assertions about what a deceased person “would have said,” without corroboration, are not enough.
The Fourth Circuit declines to create a Padilla exception to the Custis rule.
- Padilla-type immigration-advice deficiencies remain within the general category of ineffective assistance and do not permit collateral attacks on prior state convictions in later federal prosecutions.
- The only recognized exception remains convictions obtained with a complete denial of counsel.
Rule 704(b) must be read narrowly after Diaz.
- Only explicit statements that the defendant did or did not have a particular mental state are forbidden.
- Experts may offer detailed opinions about cognitive abilities, literacy, and group tendencies, even when those opinions strongly suggest a mental-state inference, so long as they do not directly state that inference.
- In Palmer, excluding grade‑level opinions under Rule 704(b) was error, but harmless because those opinions were cumulative of other admitted evidence.

B. The Case’s Place in the Broader Legal Landscape

United States v. Palmer sits at a crossroads of criminal procedure, evidence, and immigration-related criminal enforcement:

It solidifies the Fourth Circuit’s adherence to restrictive doctrines on preindictment delay and collateral attacks on prior convictions.
It provides an important, concrete example of how Diaz reshapes the boundaries of expert testimony concerning a defendant’s mental state, signalling that Rule 704(b) should not be used as a broad gatekeeping device against capacity evidence.
For noncitizen and naturalized defendants, it illustrates the high stakes of state guilty pleas: once entered and not timely challenged in the proper forum, those convictions can later support federal naturalization-fraud prosecutions and automatic loss of citizenship, with limited ability to revisit the underlying plea in federal court.

C. Final Observation

While Palmer did not obtain relief, the decision will likely influence how future defendants structure their expert evidence and how district courts apply Rule 704(b) in the wake of Diaz. It also serves as a cautionary tale on two fronts: the importance of early and proper forums for challenging potentially defective pleas, and the difficulty of relying on preindictment delay or lost witnesses as a basis for dismissing an otherwise timely federal prosecution.