Cultivating, distributing and possessing marijuana is a federal crime and marijuana businesses can't be assured they won't be prosecuted, the top federal prosecutor in Massachusetts said Monday, after legalization advocates.

"I understand that there are people and groups looking for additional guidance from this office about its approach to enforcing federal laws criminalizing marijuana cultivation and trafficking," Andrew Lelling, the US Attorney in Massachusetts, said in a statement. "I cannot, however, provide assurances that certain categories of participants in the state-level marijuana trade will be immune from federal prosecution."

US Attorney General Jeff Sessions last week rescinded an Obama administration memo that allowed a marijuana industry to flourish in states that have legalized the substance. He is reportedly leaving further action up to local US attorneys.

Massachusetts voters broadly legalized recreational marijuana through a 2016 ballot question. Medical marijuana became legal in Massachusetts after a 2012 ballot question.

The new state agency, the Cannabis Control Commission, is setting up a framework for the regulatory oversight of retail pot shops that are due to open in July 2018.

But Lelling said Congress "unambiguously" made marijuana an illegal drug.

"As a law enforcement officer in the Executive Branch, it is my sworn responsibility to enforce that law, guided by the Principles of Federal Prosecution," Lelling said in a statement. "To do that, however, I must proceed on a case-by-case basis, assessing each matter according to those principles and deciding whether to use limited federal resources to pursue it."

Acknowledging that he has been asked for details on how he plans to enforce federal laws criminalizing marijuana cultivation and trafficking, Lelling placed the ball back in Congress' court.

"Deciding, in advance, to immunize a certain category of actors from federal prosecution would be to effectively amend the laws Congress has already passed, and that I will not do," he said. "The kind of categorical relief sought by those engaged in state-level marijuana legalization efforts can only come from the legislative process."

Marijuana advocates said Sessions' move "created a procedural vacuum" and has done "no favors" to US attorneys like Lelling.

"We understand the reluctance to provide categorical answers but moving forward under a vast cloud of uncertainty is difficult for all stakeholders, including the residents of Massachusetts," Jim Borghesani, spokesman for the group behind the 2016 marijuana legalization ballot question, said in an email to MassLive.