Federal investigators have sought subpoenas for employees at the federal jail in New York City where Jeffrey Epstein killed himself, according to two sources familiar with the investigation.

Justice Department officials have said for over a week that some of the employees who were on duty when the financier and convicted sex trafficker died in his cell have retained lawyers and declined investigators' requests for interviews.

Attorney General William Barr mentioned the impasse Wednesday, saying: "Unfortunately, there have been some delays because a number of the witnesses were not cooperative." He added that the Justice Department was nonetheless "moving expeditiously," an apparent reference to the move to seek subpoenas.

Investigators were looking into whether two guards tasked with watching Epstein may have been sleeping when he died, two officials familiar with the investigation told NBC News last week. Investigators had not yet reached any conclusions as of last week, the law enforcement sources said.

Barr on Monday removed the acting director of the federal Bureau of Prisons in the wake of Epstein's suicide Aug. 10. The warden of the Metropolitan Correctional Center in Manhattan, Lamine N'Diaye, has been temporarily reassigned, and the two guardsassigned to watch Epstein have been placed on leave.

Epstein, 66, was not on suicide watch at the time of his death, multiple people familiar with the investigation have told NBC News. The FBI and the Justice Department are investigating how he was able to take his own life. Barr has said "serious irregularities" had been found at the lockup.

The politically connected billionaire was arrested in July on charges of sex trafficking of minors and conspiracy to engage in sex trafficking of minors. The arrest stemmed from incidents spanning from 2002 to 2005, three law enforcement officials previously said. Epstein had pleaded not guilty.

The news of the subpoenas comes a day after the filing of three lawsuits containing allegations that Epstein employed an army of "disciples" through a complex network of companies for many years to recruit girls and young women to have sex with him.