But on Aug. 9, the day before Mr. Epstein was found dead, his cellmate was transferred out in a “routine, prearranged transfer,” the indictment said.

That evening, Ms. Noel and Mr. Thomas were both working overtime shifts. Ms. Noel had been working as a correction officer in the Manhattan jail since 2016. Mr. Thomas started there as a correction officer in 2007 and, though he was assigned to another position within the jail in 2013, he frequently worked overtime shifts as an officer, the indictment said.

Mr. Epstein was escorted into his cell by Ms. Noel and another guard shortly before 8 p.m., according to the indictment. By 10 p.m., inmates were locked in their cells for the night. Surveillance footage taken at around 10:30 p.m. shows Ms. Noel walking up to the only door to the cluster of cells where Mr. Epstein was housed, the indictment said; over the next few hours, nobody is seen approaching the wing, including Ms. Noel and Mr. Thomas. They were supposed to check in on Mr. Epstein and other inmates every half-hour.

But instead of monitoring detainees, the two “sat at their desk, browsed the internet and moved around the common area,” the indictment said. They then signed “count sheets” saying they had checked on inmates multiple times overnight when they had not.

The next morning, when Ms. Noel and Mr. Thomas entered Mr. Epstein’s cell, they found him unresponsive “with a noose around his neck,” according to the indictment. When a supervisor arrived, the guards admitted they had not properly performed their duties.

“We did not complete the 3 a.m. nor 5 a.m. rounds,” Ms. Noel said, according to the indictment.

Mr. Thomas said, “She’s not to blame, we didn’t do any rounds.”

The director of the federal Bureau of Prisons, Kathleen Hawk Sawyer, said in a statement that the agency was taking the allegations of misconduct “very seriously” and that they “will be responded to appropriately.”