WASHINGTON -- Attorney General Jeff Sessions is seeking the resignations of 46 United States attorneys who were appointed during prior presidential administrations, the Justice Department said Friday.

Many of the federal prosecutors who were nominated by former President Barack Obama have already left their positions, but the nearly four dozen who stayed on in the first weeks of the Trump administration have been asked to leave "in order to ensure a uniform transition," Justice Department spokeswoman Sarah Isgur Flores said.

"Until the new U.S. attorneys are confirmed, the dedicated career prosecutors in our U.S. attorney's offices will continue the great work of the department in investigating, prosecuting and deterring the most violent offenders," she said in a statement.

It is customary for the country's 93 U.S. attorneys to leave their positions once a new president is in office, but the departures are not automatic. One U.S. attorney appointed by President George W. Bush, Rod Rosenstein of Maryland, remained on the job for the entire Obama administration and is the current nominee for deputy attorney general.

Tim Purdon, a former U.S. attorney for North Dakota in the Obama administration, recalled that Obama permitted Bush appointees to remain on until their successors had been appointed and confirmed.

"The way the Obama administration handled it was appropriate and respectful and classy," he said. "This saddens me because many of these people are great public servants and now they are being asked to leave."

U.S. attorneys are federal prosecutors who are nominated by the president, generally upon the recommendation of a home-state senator, and are responsible for prosecuting federal crimes in the territories they oversee. They report to Justice Department leadership in Washington, and their priorities are expected to be in line with those of the attorney general.

Sessions took perhaps a veiled swipe at their work in a memo earlier this week, saying that prosecutions for violent crime have been on the decline even as the number of murders has gone up. The demand for resignations seems a way to ensure he will have a team of new federal prosecutors more likely to share his agenda.

It was not immediately clear when each of the prosecutors would resign, or if they all actually will. And the request for resignations doesn't necessarily mean Sessions plans to accept all of them. In November, Preet Bharara, the U.S. attorney for Manhattan, said that he'd been asked by Trump to stay on and that he intended to.

Bharara's office declined to comment Friday.

Montana's U.S. Attorney Mike Cotter said he received a phone call from Acting Deputy Attorney General Dana Boente telling him "the president has directed this."

"I think it's very unprofessional and I'm very disappointed," he said. "What happened today on Friday, March 10, that was so important that all Obama appointees who are US attorneys need to be gone?"

"I gotta write that (resignation) letter. It's going to be a one-liner," he added.

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Associated Press writer Matt Volz in Helena, Montana, contributed to this report.