Justice Department officials were reluctant on Thursday to predict that a change in federal policy toward marijuana would result in aggressive new enforcement measures against an industry that has grown rapidly in the past few years.

They conceded that the shift injected uncertainty into the system sanctioned by individual states for growing and selling cannabis plants. But that very doubt appears to be what they believe is one of the benefits of the policy change.

Attorney General Jeff Sessions told the nation's U.S. attorneys on Thursday that they are free to bring marijuana cases if they believe it's a local priority in their districts. In a three-paragraph memo, he rescinded the Justice Department's previous hands-off approach taken during the Obama administration.

Related: Sessions to end legal marijuana policy from Obama era

A senior Justice Department official said the Obama-era policy "resulted in a belief that certain marijuana operators would not be prosecuted" and was perceived "to have created a safe harbor for the industry to operate."

Articulated in a 2013 memorandum, the Obama policy instructed the nation's federal prosecutors to focus only on marijuana cases that involved gangs or organized crime, sales beyond a state border, growing marijuana plants on federal land, or distribution to children.

But Sessions stopped short Thursday of instructing prosecutors to bring more cases, and senior department officials denied that the move was intended to send a message to the industry that it's going to be the target of a new enforcement push.

Related: Sessions Reviewing Whether to Crack Down on Marijuana Possession

A Justice Department statement described the new policy as "a return to the rule of law" and said it was "a return of trust and local control to federal prosecutors who know where and how to deploy Justice Department resources most effectively."

Chuck Rosenberg, who stepped down as DEA administrator in September, said the policy revision would not substantially change the course of federal prosecutions. "Many U.S. attorneys were bringing marijuana cases when they thought it was appropriate, regardless of the Obama policy," he said.

Bob Troyer, the U.S. attorney in Colorado, said his office "has already been guided by these principles in marijuana prosecutions, focusing in particular on identifying and prosecuting those who create the greatest safety threats to our communities around the state" and would "continue to take this approach."

"Jeff Sessions isn't providing any new resources," Rosenberg said. Other legal experts said fighting the nation's opioid epidemic is a much higher priority than going after an industry blessed by the states.

But advocates of greater legalization predicted that the policy change would rock marijuana growers and dealers. "It will only create chaos and confusion for an industry that is currently responsible for creating over 150,000 American jobs and generating countless millions in state tax revenue," said Erik Altieri, executive director of the pro-marijuana group NORML.

Related: Opinion: Sessions is on the wrong side of marijuana legalization

Senator Cory Gardner, a Colorado Republican, said in a tweet that the move "directly contradicts what Attorney General Sessions told me prior to his confirmation."

This reported action directly contradicts what Attorney General Sessions told me prior to his confirmation. With no prior notice to Congress, the Justice Department has trampled on the will of the voters in CO and other states. — Cory Gardner (@SenCoryGardner) January 4, 2018

Gardner threatened to hold up confirmation of Justice Department nominees as retaliation.

"I am prepared to take all steps necessary, including holding DOJ nominees, until the Attorney General lives up to the commitment he made to me prior to his confirmation," Gardner wrote.

A senior Justice Department official said Thursday's memo was not sent at the request of the White House and originated entirely inside the department.

Sessions has long been an outspoken opponent of marijuana, even as a U.S. senator. "I don't think America will be a better place when more people, especially young people, smoke pot," he said shortly after becoming attorney general.