In a statement, Lelling said that while he would examine on a “case-by-case” basis the need to use “limited federal resources” to clamp down on marijuana operations, he could not promise he would not prosecute growers or manufacturers of the drug.

His comments stood in sharp contrast to the reaction of the US attorney in Colorado, who suggested that despite Sessions’ memo, he would continue to follow the hands-off pot policy of the Obama era.

But Nancy Gertner, a retired federal judge, said that Lelling wisely avoided rhetoric that would anger his new bosses in Washington, D.C. Prosecutors serve at the pleasure of presidents, though they rarely let go of their own appointments.

“If he said, ‘I will never prosecute,’ he will be gone,” Gertner said. “If he said, ‘I’m going to make it a priority,’ he will be completely inconsistent with the priorities of this state. So what does he say? Case-by-case basis.”

Gertner said the office of the US attorney is meant to be decentralized, with prosecutors relatively free to set their priorities.

“There is no question that things are less certain than they were before,” Gertner said. “But [Lelling] has an enormous amount of discretion.”

Martin W. Healy, chief counsel of the Massachusetts Bar Association, which supported Lelling’s nomination, said his numerous conversations with Lelling have not suggested a hard-line prosecutor who will target medical marijuana dispensaries or licensed shops.

Massachusetts “got a prosecutor who works in the trenches and hasn’t been a zealot or heavy-handed in any way,” Healy said.