WASHINGTON >> The Trump administration threw the nation’s burgeoning movement to legalize marijuana into uncertainty today, terminating an Obama-era leniency policy that kept federal authorities from cracking down in states that have declared it legal. Prosecutors can now go after the pot trade whenever state and federal drug laws collide, Attorney General Jeff Sessions declared.

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. And, the officials said, Thursday’s action might not be the only step toward greater marijuana enforcement. The department has the authority to sue states on the grounds that state laws regulating pot are unconstitutional, pre-empted by federal law.

Asked about the change, White House spokeswoman Sarah Sanders said only that Trump’s top priority is enforcing federal law “and that is regardless of what the topic is, whether it’s marijuana or whether it’s immigration.”

The Obama administration in 2013 announced it would not stand in the way of states that legalize marijuana, so long as officials acted to keep it from migrating to places where it remained outlawed and keep it out of the hands of criminal gangs and children. That memo, written by then-Deputy Attorney General James M. Cole, had cleared up some of the uncertainty about how the federal government would respond as states began allowing sales for recreational and medical purposes.

But the Sessions Justice Department believed the Cole memo created a “safe harbor” for marijuana by allowing states to flout federal law, Justice Department officials said. Sessions, in his memo, called the Obama guidance “unnecessary.”

He and some law enforcement officials in states such as Colorado blame legalization for a number of problems, including drug traffickers who have taken advantage to illegally grow and ship the drug across state lines, where it can sell for much more.

Marijuana advocates argue those concerns are overblown and contend legalizing the drug reduces crime by eliminating the need for a black market. They quickly condemned Sessions’ move as a return to outdated drug-war policies that unduly affected minorities.

Sessions “wants to maintain a system that has led to tremendous injustice … and that has wasted federal resources on a huge scale,” said Maria McFarland Sanchez-Moreno, executive director of the Drug Policy Alliance. “If Sessions thinks that makes sense in terms of prosecutorial priorities, he is in a very bizarre ideological state, or a deeply problematic one.”

But the decision was a win for marijuana opponents who had been urging Sessions to take action.

“There is no more safe haven with regard to the federal government and marijuana, but it’s also the beginning of the story and not the end,” said Kevin Sabet, president and CEO of Smart Approaches to Marijuana, who was among several anti-marijuana advocates who met with Sessions last month. “This is a victory. It’s going to dry up a lot of the institutional investment that has gone toward marijuana in the last five years.”

Yet confusion remains.

Jane Stinson, part-owner of the retail marijuana shop Enlighten Alaska in Anchorage, called Thursday’s action confusing and worried that it could harm her business. The change, she said, “can have so many ripple effects we just don’t know.”

The change reflects yet another way in which Sessions, who served as a federal prosecutor at the height of the drug war in Mobile, Alabama, has reversed more lenient Obama-era criminal justice policies. While his Democratic predecessor Eric Holder told federal prosecutors to avoid seeking long mandatory minimum sentences when charging certain lower-level drug offenders, for example, Sessions issued an order demanding the opposite, telling them to pursue the most serious charges possible against most suspects.

A task force Sessions convened to study pot policy made no recommendations for upending the legal industry but instead encouraged Justice officials to keep reviewing the Obama administration’s more hands-off approach, something Sessions promised to do.