(CNN)A new Missouri state law that blocks state and local law enforcement from enforcing federal gun laws has already impeded criminal investigations, the Justice Department said in a court filing Wednesday supporting a legal effort to overturn the law.
Missouri Gov. Mike Parson, a Republican, signed the "Second Amendment Preservation Act" in June. The city of St. Louis filed a lawsuit seeking to block the law shortly afterward.
The law, which is expected to go in effect later this month, "prohibits state and local cooperation with federal officials that attempt to enforce any laws, rules, orders, or actions that violate the Second Amendment rights of Missourians," according to Parson's office.
Under the law, "protections against federal overreach are triggered if federal officials attempt to violate the state or federal constitution. Additionally, the bill is an acknowledgment that the right to keep and bear arms is fundamental and inalienable," according to the governor's office.
In its statement of interest filed with the court Wednesday, the Justice Department said: "Among other things, the new law prohibits local law enforcement from participating in federal task forces and purports to ban local police from enforcing federal gun laws."
"Under the United States Constitution's Supremacy Clause, the State of Missouri has no power to nullify federal laws," the department said.
Missouri has seen more than 8,000 firearm crimes so far this year and 218 homicides, 74% of which involved a firearm, according to a declaration from Frederic D. Winston, a seasoned Kansas City-based Bureau of Alcohol, Tobacco, Firearms and Explosives special agent in charge, that was filed with the court. Winston's role, in part, is to limit unlawful access to firearms to prevent additional violent crimes in the state.
"SAPA has negatively harmed law enforcement and public safety in Missouri," Winston wrote.
Since the passage of the law, according to Winston, a dozen state and local officers have withdrawn from collaborative task forces.
"By withdrawing state and local officials from ATF task forces, ATF is no longer able to fulfill its duties as effectively, including preventing, investigating and assisting in the prosecution of violent offenders. These state and local officials are critical members of ATF's law enforcement efforts," Winston wrote.
In addition, according to Winston, the Missouri Information Analysis Center, which collects reports of suspicious activities to identify potential terrorist or criminal operations, has told the ATF that it will no longer provide investigative support to the agency, and several state and local law enforcement agencies say they'll no longer put data into an interstate ballistic imaging network.
A hearing is scheduled for Thursday on the underlying lawsuit.