Federal prosecutors have subpoenaed up to 20 staff members at the Metropolitan Correctional Center in New York, where Jeffrey Epstein killed himself while awaiting trial on charges of sexually abusing teenage girls.

Epstein had pleaded not guilty and was preparing to make the argument that he could not be charged because of a 2008 deal he made to avoid federal prosecution on similar allegations. He was taken off suicide watch about a week after he was found with bruises around his neck, and put back in a high-security housing unit where he was supposed to be checked on every 30 minutes.

The wealthy financier was found dead on Aug. 10, ending the possibility of a trial that would have involved celebrities, academics, and powerful politicians, and putting pressure on Attorney General William Barr to explain how such a prominent defendant was left unsupervised long enough to hang himself.

Since Epstein’s death, Barr has removed the director of the federal Bureau of Prisons, reassigned the jail’s warden to a desk post at a regional office. Two guards, who were supposed to be watching Epstein the night he died, are accused of falsely recording in a log that they had failed to check on Epstein every 30 minutes, as required.

Barr, who has said investigators found “serious irregularities” at the jail, confirmed Wednesday that the FBI’s investigation has been slowed because some witnesses have been uncooperative. The subpoenas will seek testimony from jail employees.

Barr said that witnesses at the jail were requiring union representation and lawyers before they would agree to interviews.

Includes reporting by the Associated Press.