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The same day that Minnesota Gov. Tim Walz signed a bill legalizing marijuana, federal authorities issued a warning to gun owners here: Federal law prohibits cannabis users from owning firearms, even if the drug is legal in their state.

Marijuana is still a Schedule I drug federally — a category that also includes heroin, LSD and ecstasy. The federal Gun Control Act of 1968 prohibits users of a controlled substance from "shipping, transporting, receiving, or possessing firearms or ammunition," according to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).

"Until marijuana is legalized federally, firearms owners and possessors should be mindful that it remains federally illegal to mix marijuana with firearms and ammunition," Jeff Reed, ATF's acting special agent in charge of the St. Paul field division, said in a statement. "As regulators of the firearms industry and enforcers of firearms laws, we felt it was important to remind Minnesotans of this distinction as the marijuana laws adjust here."

It's an issue that has surfaced in every state that has legalized marijuana for medical or recreational use. In Minnesota, medical cannabis advocates, gun-rights supporters and some state lawmakers have pushed for solutions to the federal problem.

A federal judge in Oklahoma ruled in February that it is unconstitutional for the government to bar marijuana users from owning guns. The federal government has appealed the ruling.

Minnesota's new marijuana law provides some firearms protections. It prohibits sheriffs from denying permits to carry based solely on legal cannabis use, and it bars state or local agencies from asking about cannabis use "for the purpose of approving or disapproving the person from purchasing, owning, possessing or carrying a firearm."

But Rob Doar, vice president of the Minnesota Gun Owners Caucus, said Minnesotans should still be cautious about mixing firearm ownership and marijuana use.

"It is clear that the ATF sending out that letter means that they're certainly monitoring the situation," Doar said. "If you do recreationally or medically use cannabis, you should really be aware that at least as far as the federal government is concerned, that you're putting your ability to own and possess firearms at risk."

Doar wants Minnesota to take further action to protect gun owners who use marijuana. The state should prohibit its agencies from prosecuting Minnesotans who violate the federal law, he said.

"It's kind of like what sanctuary cities do," Doar said. "Walz could even issue an executive order to that effect, to provide some comfort for gun owners."

It remains unclear whether federal authorities will actively enforce this law once cannabis use and possession become legal in Minnesota.

In nearly a decade since Minnesota legalized medical marijuana, local cannabis attorney Jason Tarasek said he's "never heard of anyone losing their gun rights because they're using cannabis."

But Tarasek said the ATF's recent bulletin could have a "chilling effect" on gun owners who are interested in using cannabis.

In Michigan, the first Midwestern state to legalize marijuana, local law enforcement agencies have sometimes enforced this law, but typically only for people they were already investigating, said Rick Thompson, executive director of the Michigan chapter of the National Organization for the Reform of Marijuana Laws (NORML).

"It's one of those things that everyone that uses cannabis is aware of. But unless you run afoul of the law, there's no problem," Thompson said.

Urban prosecutors in Michigan have shown more willingness to forgive the federal crime of cannabis users possessing firearms, while "zealous" rural prosecutors have been more likely to charge people for it, Thompson said.

But if you're a law-abiding gun owner who uses cannabis at home, Thompson said there's no reason law enforcement should find out about it.

"The only time this ever becomes a problem is when illegal activity is discovered," he said. "So, as long as cannabis users are complying fully with all state laws, they probably will not ever experience a problem with this."