The reversal added to confusion about whether it’s OK to grow, buy or use marijuana in states where pot is legal, since long-standing federal law prohibits it. And it caused concern that prosecutors would feel empowered to jail individuals for marijuana possession.

“I am not going to tell Colorado or California or someone else that possession of marijuana is legal under United States law,” Sessions said, answering student questions after the speech at Georgetown’s law school. But, he added, federal prosecutors “haven’t been working small marijuana cases before, they are not going to be working them now.”