U.S. District Judge Brian Wimes ruled the 2021 law is preempted by the federal government under the U.S. Constitution’s supremacy clause.

“At best, this statute causes confusion among state law enforcement officials who are deputized for federal task force operations, and at worst, is unconstitutional on its face,” Wimes wrote.

Missouri’s Republican Attorney General Andrew Bailey in a statement said he will appeal the ruling.

“As Attorney General, I will protect the Constitution, which includes defending Missourians’ fundamental right to bear arms,” Bailey said. “We are prepared to defend this statute to the highest court, and we anticipate a better result at the Eighth Circuit.”

The Missouri law had subjected law enforcement agencies with officers who knowingly enforced federal gun laws without equivalent state laws to a fine of $50,000 per violating officer.

Federal laws without similar Missouri laws include statutes covering weapons registration and tracking, and possession of firearms by some domestic violence offenders.

Conflict over Missouri’s law wrecked a crime-fighting partnership with U.S. attorneys that Missouri’s former Republican attorney general, now-Sen. Eric Schmitt, touted for years. Under Schmitt’s Safer Streets Initiative, attorneys from his office were deputized as assistant U.S. attorneys to help prosecute violent crimes.

The Justice Department, which last year sued to overturn the Missouri law, said the Missouri state crime lab, operated by the Highway Patrol, refused to process evidence that would help federal firearms prosecutions after the law took effect.

The Missouri Information and Analysis Center, also under the Highway Patrol, stopped cooperating with federal agencies investigating federal firearms offenses. And the Highway Patrol, along with many other agencies, suspended joint efforts to enforce federal firearms laws.

Wimes said police can now work with federal partners without worrying about breaking the voided law.

“State and local law enforcement officials in Missouri may lawfully participate in joint federal task forces, assist in the investigation and enforcement of federal firearm crimes, and fully share information with the Federal Government without fear of H.B. 85’s penalties,” the judge wrote.

Several Missouri prosecutors had testified against the bill, saying it jeopardized investigations and prosecutions against serious offenders while exposing state and local officers to civil liability.

“Given today’s ruling, Missouri’s prosecutors and our state and local law enforcement partners look forward to again utilizing federal assistance when appropriate to apprehend and punish violent offenders who endanger the citizens of our state,” the statement said. “Police and prosecutors will return to their work without fear of losing their jobs or being held liable personally to criminal offenders simply because they are doing vital work to keep our communities safe.”

Republican lawmakers who helped pass the bill said they were motivated by the potential for new gun restrictions under Democratic President Joe Biden, who signed the most sweeping gun violence bill in decades last year.

The federal legislation toughened background checks for the youngest gun buyers, keeps firearms from more domestic violence offenders and helps states put in place red flag laws that make it easier for authorities to take weapons from people adjudged to be dangerous.

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Associated Press writer Heather Hollingsworth and Margaret Stafford contributed to this report from Kansas City, Missouri.