Colleen McMahon, the chief judge in New York's southern district, sent a letter last week to fellow judges laying out the prospect of a possible appointment falling to judges in her district after Attorney General Jeff Sessions named a new acting US attorney for Manhattan.
Geoffrey Berman replaced Joon Kim, who held the job on an interim basis after President Donald Trump last year fired Preet Bharara, an Obama appointee who Trump earlier had offered to keep in the job.
McMahon, a Clinton administration appointee, noted in her letter that under the law Berman's period as acting U.S. attorney is limited to 120 days. By the judge's calculation, that means that by early May, the court may be called on to appoint another interim US attorney, a rare occurrence.
"If (as appears likely) the office of United States Attorney remains vacant on that date, the Judiciary Act provides that this court may name someone to fill the office until such time as the President nominates and Congress confirms a new incumbent," she wrote in her letter, a copy of which was reviewed by CNN.
McMahon's letter was met with surprise among other judges, according to court sources who spoke to CNN. But it appears to reflect the Trump administration's slow pace in filling key Justice Department jobs
The Justice Department didn't comment on the judge's letter.
Berman is a former prosecutor in the office and is well-regarded. He is widely expected to eventually become the Trump nominee for the job.
The Manhattan US attorney post is considered one of the Justice Department's premier prosecutor jobs. It has oversight over some of the nation's most high-profile investigations, and it has jurisdiction over matters related to Trump's businesses.
Given ongoing investigations related to the Trump campaign and the unusual firing of Bharara, the administration's moves to fill the job are being closely followed.
McMahon said in her letter that Berman "assured me that he was proud to be returning to the office and would never do anything to harm its reputation."
She added that, "I have no reason to believe that the new acting US Attorney will do anything that would occasion the imminent, wholesale departure of the senior staffers with whom the court has worked over the past nine years, and I certainly hope that will not happen."