State Environmental Enforcement Claims May Be Severed and Remanded After Third-Party Federal-Officer Removal; Law-of-the-Case Bars Renewed “Acting Under” Theory

I. Introduction

In Mich. Dep't of Env't, Great Lakes, & Eng. v. Gerald R. Ford Int'l Airport (6th Cir. June 30, 2026), the Sixth Circuit addressed a recurring procedural problem in PFAS litigation: when a State brings a purely state-law environmental enforcement action, but the defendant later impleads product manufacturers who remove under the federal-officer removal statute to reach the federal AFFF multidistrict litigation (“MDL”).

The plaintiffs-appellees were the Michigan Department of Environment, Great Lakes, and Energy and Michigan Attorney General Dana Nessel on behalf of the People of Michigan (collectively, the “State”). The defendant-appellant was the Gerald R. Ford International Airport Authority (“Airport Authority”). The State sued in Michigan court under Michigan’s Natural Resources and Environmental Protection Act (“NREPA”), alleging PFAS contamination tied to the Airport’s historical use of aqueous film-forming foam (“AFFF”) and related permit violations.

The core issues on appeal were procedural and jurisdictional:

  • whether the district court abused its discretion by severing the State’s claims from the Airport Authority’s third-party claims against AFFF manufacturers;
  • whether the court properly declined supplemental jurisdiction and remanded only the severed State claims to state court;
  • whether the Airport Authority could resurrect its own federal-officer removal theory after losing that issue in a prior Sixth Circuit appeal.

II. Summary of the Opinion

The Sixth Circuit affirmed across the board:

  1. Severance affirmed. Applying the case-management discretion recognized in Federal Rules of Civil Procedure 21 and 14, and the multi-factor framework from Parchman v. SLM Corp., the court held it was reasonable to sever the State’s direct NREPA enforcement claims from manufacturer-focused third-party claims.
  2. Declining supplemental jurisdiction affirmed. Under 28 U.S.C. § 1367(c), the district court permissibly declined to exercise supplemental jurisdiction over the severed State claims because state-law issues predominated and because exceptional circumstances supported remand—especially given the “procedural loop” of a once-remanded enforcement action returning to federal court solely through third-party removal.
  3. Renewed federal-officer removal rejected via law of the case. The Airport Authority’s attempt to re-argue that it was “acting under” the FAA for purposes of 28 U.S.C. § 1442(a)(1) was barred by law of the case, based on the Sixth Circuit’s earlier decision in Mich. Dep't of Env't, Great Lakes, & Energy v. Gerald R. Ford Int'l Airport Auth., No. 24-1085, 2024 WL 4867042. The intervening authority Chevron USA Inc. v. Plaquemines Parish did not alter the “acting under” element.

III. Analysis

A. Precedents Cited

1. The prior removal appeal and “acting under” (law of the case)

The decisive constraint on the Airport Authority’s jurisdictional strategy was the Sixth Circuit’s prior holding in Mich. Dep't of Env't, Great Lakes, & Energy v. Gerald R. Ford Int'l Airport Auth., No. 24-1085, 2024 WL 4867042. There, the court held that FAA requirements and grant assurances reflected a regulatory relationship, not the kind of delegation/supervision that makes a private party “acting under” a federal officer.

In the 2026 opinion, the court treated that earlier “acting under” determination as binding under law-of-the-case principles, relying on In re Kenneth Allen Knight Tr. (issues decided explicitly or by necessary implication), Hanover Ins. Co. v. Am. Eng'g Co. (limited exceptions), and Christianson v. Colt Industries Operating Corp. (law-of-the-case applies with full force even when jurisdiction is implicated). The court also acknowledged, consistent with Doe v. Univ. of Mich., that subject-matter jurisdiction can sometimes be revisited—but found no qualifying change here.

The Airport Authority pointed to Chevron USA Inc. v. Plaquemines Parish, which relaxed the causal nexus under § 1442(a)(1)’s “relating to” language. The Sixth Circuit explained why that did not help: Chevron may speak to the second element (the connection between the conduct and federal office), but the earlier appeal failed at the first element—whether the Airport Authority was “acting under” a federal officer at all.

2. Severance doctrine

The severance analysis was anchored in Parchman v. SLM Corp., which supplies non-exhaustive factors: same transaction or occurrence; common questions; judicial economy; prejudice; and witnesses/proof. The Sixth Circuit emphasized that these factors guide, but do not displace, the broad discretion conferred by Rule 21.

The Airport Authority relied on Defense Distributed v. Bruck and Sunbelt Corp. v. Noble, Denton & Associates, Inc., where severance had split claims a plaintiff chose to bring together. The court distinguished those cases on a critical procedural axis: Michigan (the plaintiff) sought to sever its own claims from later-filed third-party claims; the plaintiff was not being forced to litigate a unified pleading in fragments against its will.

3. Supplemental jurisdiction and remand discretion

For supplemental jurisdiction, the court relied on:

  • Gamel v. City of Cincinnati (abuse-of-discretion review; discretion under § 1367(c));
  • Habich v. City of Dearborn and Baer v. R & F Coal Co. (supplemental jurisdiction is discretionary, not a right);
  • Carnegie-Mellon Univ. v. Cohill (economy, convenience, fairness, comity);
  • Rose v. Hartford Underwriters Ins. Co. (a court must give reasons to show it exercised discretion).

On the remand/jurisdiction standards, the court cited Mays v. City of Flint and Smith v. Nationwide Prop. & Cas. Ins. Co. for de novo review of subject-matter jurisdiction and remand predicated on lack of jurisdiction.

4. MDL context

The decision also situates itself within the AFFF MDL’s purpose, referencing In re Aqueous Film-Forming Foams Prods. Liab. Litig., which centralized thousands of manufacturer-focused cases involving toxicity, manufacturer knowledge, warnings, and concealment. That framing mattered to the severance and comity analysis: the MDL’s efficiency rationale is strongest for manufacturer-liability questions, not necessarily for a State’s direct enforcement action against a local operator under state statutory duties.

B. Legal Reasoning

1. Severance: overlap acknowledged, but not dispositive

The Sixth Circuit accepted the Airport Authority’s central factual point: both the State’s claims and the third-party claims share some overlap around the source and movement of PFAS. The district court even found the “same transaction or occurrence” factor did not favor severance. But the appellate panel stressed that overlap does not control the Rule 21 decision.

The key distinction was issue density and proof divergence:

  • The State’s NREPA claims focus on whether the Airport Authority caused or allowed releases/threatened releases (Part 201) and violated its NPDES permit (Part 31).
  • The Airport Authority’s third-party claims—contribution/cost recovery, indemnification, products liability, negligence, trespass—invite manufacturer-specific inquiries (design, warnings, knowledge, concealment, arranger liability) that are not necessary to adjudicate the State’s direct enforcement case.

The court also treated timing and conditionality as relevant: contribution may occur “during or following” the underlying action, and indemnification is contingent on the Airport Authority being held liable. That structure supported the practical case-management judgment to let the State’s enforcement claims proceed on their own track.

2. Supplemental jurisdiction: severance reshaped the jurisdictional equities

The Sixth Circuit’s most important doctrinal move is recognizing how severance changes the jurisdictional “picture.” Once the State’s claims were severed, the only federal hook keeping the case in federal court stemmed from the removing third-party manufacturers’ federal-officer theory (manufacturing to military specifications). The State’s claims, by contrast, were purely Michigan-law enforcement claims.

The district court’s written discussion was “brief,” but sufficient under Rose v. Hartford Underwriters Ins. Co. because it gave intelligible reasons: state-law predominance and exceptional circumstances/compelling reasons (including the earlier remand and likely MDL delay). The Sixth Circuit treated the unusual procedural loop—return to federal court only through third-party removal after the State had already won remand—as a legitimate “exceptional circumstances” consideration under § 1367(c)(4), and also endorsed the “substantially predominate” rationale under § 1367(c)(2).

3. Federal-officer removal: law of the case as a jurisdictional backstop

The court’s refusal to revisit the Airport Authority’s FAA-based § 1442(a)(1) theory underscores that law-of-the-case can function as a stabilizing device even in jurisdictional disputes. While jurisdiction can be revisited in appropriate circumstances, the Sixth Circuit found none of the recognized exceptions: no materially different evidence, no intervening controlling authority on the “acting under” element, and no clear error causing manifest injustice.

The court’s treatment of Chevron USA Inc. v. Plaquemines Parish is especially clarifying: changes to the “relating to” element do not matter if the party still cannot satisfy the threshold “acting under” element.

C. Impact

  • State enforcement actions gain protection from MDL gravitational pull. The decision provides a road map for keeping a State’s direct enforcement claims in state court even when manufacturer third-party claims are removed to federal court and funneled to an MDL.
  • District courts are affirmed as active docket managers in removal/MDL-adjacent settings. By emphasizing Rule 21 discretion and endorsing a severance-plus-§ 1367(c) approach, the Sixth Circuit validates a pragmatic sequencing: adjudicate direct state-law regulatory claims without waiting for manufacturer-centric MDL proceedings.
  • Law-of-the-case limits repetitive federal-officer removal arguments. Parties who lose the “acting under” issue on appeal should expect strong preclusion absent a genuinely new factual record or intervening authority that addresses the same element.
  • Clarification after Chevron: the opinion signals that litigants should not overread Chevron. Even if “relating to” is broader, the “acting under” gateway remains a distinct, potentially dispositive barrier.

IV. Complex Concepts Simplified

  • Federal-officer removal (28 U.S.C. § 1442(a)(1)): lets certain defendants move a case from state to federal court when they were assisting or carrying out duties for the federal government. It is not enough to be heavily regulated; the defendant must be helping the government perform a task under supervision or delegation (the “acting under” requirement).
  • Severance (Rule 21; also referenced by Rule 14(a)(4)): the court can split claims into separate actions to manage complexity, avoid unfairness, or improve efficiency—even when some facts overlap.
  • Supplemental jurisdiction (28 U.S.C. § 1367): allows federal courts to hear related state-law claims alongside federal claims, but the court may decline when state issues predominate or when compelling reasons (like comity and fairness) favor state court.
  • MDL (multidistrict litigation): a federal procedure that consolidates many similar federal cases for pretrial proceedings. It is designed to streamline common issues (here, manufacturer knowledge, warnings, product design), but can slow uniquely local enforcement disputes.
  • Law of the case: once an appellate court decides an issue, that decision generally governs later phases of the same case unless narrow exceptions apply.

V. Conclusion

The Sixth Circuit’s decision validates a two-step mechanism for preserving the State’s chosen forum in environmental enforcement litigation: (1) sever the State’s direct statutory claims from manufacturer-oriented third-party claims, and (2) decline supplemental jurisdiction and remand the severed State claims when state law predominates and exceptional circumstances support comity and efficient adjudication.

Equally significant, the court treats the “acting under” component of federal-officer removal as a stable, litigated threshold—one not easily reopened after an earlier appellate loss, and not altered merely because later Supreme Court authority broadens a different element of § 1442(a)(1).