WASHINGTON (AP) — The Trump administration threw the burgeoning movement to legalize marijuana into uncertainty Thursday as it lifted an Obama-era policy that kept federal authorities from cracking down on the pot trade in states where the drug is legal. Attorney General Jeff Sessions will now leave it up to federal prosecutors to decide what to do when state rules collide with federal drug law.

Sessions’ action, just three days after a legalization law went into effect in California, threatened the future of the young industry, created confusion in states where the drug is legal and outraged both marijuana advocates and some members of Congress, including Sessions’ fellow Republicans. Many conservatives are wary of what they see as federal intrusion in areas they believe must be left to the states.

Republican Sen. Cory Gardner, who represents Colorado, one of eight states that have legalized marijuana for recreational use, said the change contradicts a pledge Sessions made to him before being confirmed as attorney general. Gardner promised to push legislation to protect marijuana sales, saying he was prepared “to take all steps necessary” to fight the change, including holding up the confirmation of Justice Department nominees. Another Republican senator, Lisa Murkowski of Alaska, called the announcement “disruptive” and “regrettable.”

Colorado’s U.S. attorney, Bob Troyer, said his office won’t change its approach to prosecution, despite Sessions’ guidance. Prosecutors there have always focused on marijuana crimes that “create the greatest safety threats” and will continue to be guided by that, Troyer said.

The largely hands-off approach to marijuana enforcement set forth by Barack Obama’s Justice Department allowed the pot business to flourish into a sophisticated, multimillion-dollar industry that helps fund some state government programs. What happens now is in doubt.

“In deciding which marijuana activities to prosecute under these laws with the Department’s finite resources, prosecutors should follow the well-established principles that govern all federal prosecutions,” considering the seriousness of a crime and its impact on the community, Sessions told prosecutors in a one-page memo.

While Sessions, a longtime marijuana foe, has been carrying out a Justice Department agenda that follows Trump’s top priorities on such issues as immigration and opioids, this change reflects his own concerns. He railed against marijuana as an Alabama senator and has assailed it as comparable to heroin.

Trump, as a candidate, said pot should be left up to the states, but his personal views on marijuana remain largely unknown.

It is not clear how the change might affect states where marijuana is legal for medical purposes. A congressional amendment blocks the Justice Department from interfering with medical marijuana programs in states where it is allowed. Justice officials said they would follow the law, but would not preclude the possibility of medical-marijuana related prosecutions.

Officials wouldn’t say whether federal prosecutors would target marijuana shops and legal growers, nor would they speculate on whether pot prosecutions would increase.

They denied the timing was connected to the opening of California sales, which are projected to bring in $1 billion annually in tax revenue within several years.