Medical marijuana states received some chilling news on Monday.

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Attorney General Jeff Sessions has asked Congress to let a policy expire that protects states that have legalized marijuana from federal interference, when it comes up for renewal later this year. In a letter sent to congressional leaders last month—which was obtained by MassRoots—Sessions reaffirmed the Justice Department's "opposition" to the protective measure.

The protective policy that Sessions seeks to let expire is the the Rohrabacher-Farr amendment—a provision attached to an omnibus spending bill that prevents the Justice Department from using federal funds to enforce prohibition in states that have legalized medical marijuana. The rider doesn't change the legal status of cannabis under federal law, but marijuana reform advocates viewed its passage as a step in the right direction nonetheless.

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"I believe it would be unwise for Congress to restrict the discretion of the Department to fund particular prosecutions, particularly in the midst of an historic drug epidemic and potentially long-term uptick in violent crime," Sessions wrote. "The Department must be in a position to use all laws available to combat the transnational drug organizations and dangerous drug traffickers who threaten American lives."

But while the provision restricts funds for federal intervention in legal states, there are already certain exceptions that empower the Justice Department to act.

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Former Deputy Attorney General James Cole listed eight circumstances that warrant federal intervention in legal states in a 2013 document known as the Cole memo. If there's evidence that legal marijuana sales are being diverted to criminal organization, that cannabis is being trafficked across state lines, or that legal products are being sold to minors, for example, the Justice Department can use federal funds to prosecutors offenders, in spite of the protective rider.

In March, Sessions said that he "might have some different ideas" to add to the Cole memo, but that he considered "much of" the document "valid." That endorsement gave some legalization advocates a sense of relief after months of uncertainty about how President Trump's administration would approach state marijuana laws. But the emergence of his letter to Congress has raised new questions about the fate of legalized marijuana under Trump.

It remains to be seen whether Congress will abide by Sessions request, though. The rider has enjoyed bipartisan support for three years and Americans on both sides of the aisle overwhelmingly oppose to federal intervention in legal marijuana states, according to a 2017 Quinnipiac poll.

Still, the attorney general's letter represents a troubling development for legalization advocates. Sessions has previously criticized the Obama administration over its passive approach to state-level marijuana reform efforts and he's expressed interest in increasing federal enforcement in legal states. And if the protective provision isn't renewed, the task of cracking down on legal states would be much easier.