Attorney General Jeff Sessions moved Thursday to revoke the Obama-era see-no-evil policy on federal enforcement of marijuana laws, announcing that federal law enforcement will no longer look the other way even in states that have legalized use of the drug.

The decision doesn’t change the state policies, but it does give U.S. attorneys the freedom to decide how strongly to enforce federal law that classifies marijuana as an illegal substance.

Major pot producers and sellers who come to the attention of federal prosecutors are most likely to feel the effects.

SEE ALSO: Gardner lashes out at DOJ move to rescind non-interference pot policy

Mr. Sessions, in a statement, characterized the decision as a “return to the rule of law.”

“Today’s memo on federal marijuana enforcement simply directs all U.S. attorneys to use previously established prosecutorial principles that provide them all the necessary tools to disrupt criminal organizations, tackle the growing drug crisis, and thwart violent crime across our country,” he said.

But legal analysts said the move makes an already confusing situation more uncertain.

“This is something where we are going to have to wait a few weeks or months before we know what the practical effect will be,” said Alex Kreit, a University of San Diego law professor who has authored multiple books on drug policy.

The Justice Department didn’t provide much clarity.

A department official, briefing reporters on the condition of anonymity, said federal prosecutors are now free to pursue “all of the cases they feel need to be brought,” but declined to discuss exactly how distributors will be impacted.

Robert Mikos, who teaches drug law at Vanderbilt University, said those with marijuana businesses need to pay attention to the signals sent from prosecutors.

“If you are operating in the Southern District of California, for example, I would listen to the U.S. attorney for that district,” he said. “But there is not a lot the industry players can do to protect themselves if there is a crackdown. States would have to take steps to soften the blow of this.”

Mr. Sessions made the announcement just after California’s policy legalizing recreational use of marijuana took effect Jan. 1. Nevada, Washington, Colorado, Oregon, Alaska and the District of Columbia had been allowing recreational use, while Maine and Massachusetts were figuring out how to implement referendums to legalize use.

California is expected to become the nation’s largest recreational pot market, with predictions that it will pump $5 billion into the local economy and generate more than $1 billion in tax revenue over the next few years.

All told, the market in states that have legalized medical or recreational marijuana is estimated to be $6 billion and forecast to grow to $9 billion by the end of the year, according to Marijuana Business Daily, an industry trade publication. The paper estimates there are 4,500 medical and recreational shops across the country.

Politicians in states where recreational marijuana use is legal were quick to cite the economic impact of Mr. Sessions’ policy. Gov. Kate Brown, Oregon Democrat, said the federal plan would be disruptive to a state that collected $108 million in marijuana tax revenue last year.

Mr. Sessions’ decision could pump the brakes on states looking to join the ranks of legalization — though analysts said a lot depends on how prosecutors move forward.

Federal prosecutors have limited resources that are allocated toward more serious crimes such as murder and gang violence, so the chances that they will go after marijuana business owners are slim, Mr. Kreit said. But that should not stop business owners from becoming nervous.

“The odds that any one individual operator will end up before a federal prosecutor is pretty slim, but even if there is a 5 percent chance, you should worry,” he said.

Republicans and Democrats in states that have led on legalization blasted Mr. Sessions’ decision, saying he was violating his own fealty to states’ rights.

Sen. Cory Gardner, Colorado Republican, said Mr. Sessions had personally assured him that the Obama-era rules wouldn’t be rolled back. Mr. Gardner threatened to block confirmation of Justice Department nominees unless Thursday’s decision is reversed.

“With no prior notice to Congress, the Justice Department has trampled on the will of the voters in Colorado and other states,” Mr. Gardner said on Twitter.

Support for legal marijuana use has grown dramatically. Just a third of Americans supported legalization at the beginning of the last decade, but that flipped to a majority in 2013 and stood at 64 percent last year, according to Gallup polling.

Recognizing those changing views, the Obama administration moved to alter enforcement of federal law, issuing three memos announcing a hands-off enforcement policy in states that legalize marijuana as long as it is kept out of the hands of criminal gangs and minors. The most notable of those memos was authored in 2013 by Deputy Attorney General James M. Cole.

It was widely expected that Mr. Sessions would ramp up enforcement of federal marijuana law.

Last year, he issued guidance telling prosecutors to pursue the most serious charges possible against drug offenders. It reversed a policy of his predecessor, Eric H. Holder Jr., who told prosecutors to avoid long mandatory sentences.

Erik Altieri, executive director of the National Organization for the Reform of Marijuana Laws (NORML), an advocacy group for legalized weed, described Mr. Sessions’ plan as “warped.”

“The American people will not just sit idly by while he upends all the progress that has been made in dialing back the mass incarceration fueled by marijuana arrests and destabilizes an industry that is now responsible for over 150,000 jobs,” Mr. Altieri said.

Inimai Chettiar, director of the New York University Brennan Center for Justice, called the new policy a “misguided revival of ‘war on drugs.’”

He said it will divert tax dollars from more serious crimes including opioid addiction, murder and violence.

“This decision could jeopardize public safety in the long run,” Ms. Chettiar said.

But anti-drug groups said Mr. Sessions’ policy could head off a surge in pot use.

“This is a good day for public health,” said Kevin Sabet, a former Obama administration drug policy adviser who heads Smart Approaches to Marijuana.

“DOJ’s move will slow down the rise of Big Marijuana and stop the massive infusion of money going to fund pot candies, cookies, ice cream and other kid-friendly pot edibles,” he said.