alt Hacker NewsYes, but In re Neagle (1890) is SCOTUS precedent granting federal agents immunity from state criminal prosecution for acts committed while carrying out their official duties (and the act at question in that case was homicide). Now, its precise boundaries are contested - in Idaho v. Horiuchi (2001), the 9th Circuit held that In re Neagle didn’t apply if the federal agent used unreasonable force - but that case was rendered moot when the state charges were dropped, and hence the issue never made it to SCOTUS. Considering the current SCOTUS majority’s prior form on related topics (see Trump v. United States), I think odds are high they’ll read In re Neagle narrowly, and invalidate any state criminal prosecution attempts.
In re Neagle (while, unfortunately, it does not state as clear of a rule as Horiuchi on the standard that should be applied) conducts an expansive facts-based analysis on the question of whether, in fact, the acts performed were done in in the performance of his lawful federal duties (if anything, the implicit standard seems less generous to the federal officer than Horiuchi’s explicit rule, which would allow Supremacy Clause immunity if the agent had an actual and objectively reasonable belief that he acted within his lawful duties, even if, in fact, he did not.)
But, yeah, any state prosecutions (likely especially the first) is going to (1) get removed to federal court, and (2) go through a wringer of federal litigation, likely reaching the Supreme Court, over Supremacy Clause immunity before much substantive happens on anything else.
OTOH, the federal duty at issue in in re Neagle was literally protecting the life of a Supreme Court justice riding circuit, as much as the present Court may have a pro-Trump bias, I wouldn't count on it being as strong of a bias as it had in Neagle.