(CNN) Attorney General Jeff Sessions on Thursday rescinded a trio of memos from the Obama administration that had adopted a policy of non-interference with marijuana-friendly state laws.

The move essentially shifts federal policy from the hands-off approach adopted under the previous administration to unleashing federal prosecutors across the country to decide individually how to prioritize resources to crack down on pot possession, distribution and cultivation of the drug in states where it is legal.

While many states have decriminalized or legalized marijuana use, the drug is still illegal under federal law, creating a conflict between federal and state law. Thursday's announcement is a major decision for an attorney general who has regularly decried marijuana use as dangerous.

In a written statement Thursday, Sessions called the shift a "return to the rule of law" but he did not go as far as some advocates had feared he might, stopping short of explicitly directing more prosecutions, resources or other efforts to take down the industry as a whole.

"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," Sessions said in a memo to all federal prosecutors. "These principles require federal prosecutors deciding which cases to prosecute to weigh all relevant considerations of the crime, the deterrent effect of criminal prosecution, and the cumulative impact of particular crimes on the community."

The former senior Justice Department official behind the decision to harmonize federal prosecutions with state legalization efforts during the Obama told CNN in a phone interview Thursday that it's uncertain how Sessions' new memo will play out at the state level.

"The whole point was to do what we could to maintain some control in this area," said Jim Cole, former deputy attorney general and now a partner at Sidley Austin in Washington.

Marijuana legalization by the numbers

Back in 2013, as an increasing number of states began to legalize marijuana, Cole released a directive to federal prosecutors that essentially adopted a policy of non-interference with marijuana-friendly state laws.

In what became colloquially known as the "Cole memo," the department recognized that the drug was still illegal under the federal Controlled Substances Act but gave federal prosecutors permission to focus their resources elsewhere, so long as the states didn't threaten other federal priorities, such as preventing the distribution of the drug to minors and targeting cartels.

"The memo set out harms we saw associated with marijuana" but essentially said that otherwise "let's let the states deal with this," Cole told CNN. "Given a non-perfect situation, we figured this was the best way to deal with it."

The new memo likely "reduces the level of comfort in the industry until it sees how US attorneys actually implement it," Cole added. "Each US attorney now gets to decide what will and will not be prosecuted. We'll have to see how it plays out. ... There was a previously a higher level of reliability that you could operate your industry if you followed certain rules. That's not necessarily being destroyed, but it is being thrown into question."

The US Attorney's Office in Colorado released a statement Thursday saying there are no plans to change marijuana prosecutions:

"Today the Attorney General rescinded the Cole Memo on marijuana prosecutions, and directed that federal marijuana prosecution decisions be governed by the same principles that have long governed all of our prosecution decisions. The United States Attorney's Office in Colorado 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. We will, consistent with the Attorney General's latest guidance, continue to take this approach in all of our work with our law enforcement partners throughout Colorado."

Congress, industry alarmed

Sessions' shift at the Justice Department comes days after marijuana became officially legal under laws in California, the largest state. Voters in California approved the measure in November 2016, but the legal, commercial sale of marijuana under state law just went into effect with the new year.

A majority of states allow the use of medical marijuana and eight, including the entire West Coast and the District of Columbia, allow recreational use.

When asked whether the Justice Department was considering suing states that attempt to legalize the drug after this new policy has gone into effect, one senior Justice official said, "Further steps are still under consideration."

The immediate reaction to Thursday's news from the marijuana industry and some members of Congress was alarm.

Colorado Sen. Cory Gardner, a Republican, tweeted that the issue "must be left up to the states," ran counter to what he had been previously told by Sessions and threatened to hold up confirmation of DOJ nominees.

Democratic Sen. Ron Wyden from Oregon, where marijuana is also legal, similarly blasted the move.

"Trump promised to let states set their own marijuana policies. Now he's breaking that promise so Jeff Sessions can pursue his extremist anti-marijuana crusade. Once again the Trump administration is doubling down on protecting states' rights only when they believe the state is right," Wyden said in a statement.

One issue that may be potentially litigated is how the new memo affects medical versus recreational marijuana use.

Congress voted in its last session to extend a spending provision known as the Rohrabacher-Blumenauer amendment, which blocks the Justice Department from using federal funds to impede the implementation of state medical marijuana laws.

Sessions' new memo does not explicitly set forth how prosecutors should treat medical marijuana, though a senior Justice official explained that prosecutors wouldn't do anything contrary to any current federal law. The open question is how broadly or narrowly that appropriations rider may be interpreted down the line, as it is an unsettled issue in the federal courts.