UNITED States federal investigators have launched a probe into the death of financier Jeffrey Epstein following his apparent suicide in his prison cell.

Epstein, who was accused of orchestrating a sex trafficking ring and sexually abusing dozens of underage girls, was found unresponsive in his cell in the Metropolitan Correctional Center on Saturday morning and later pronounced dead in hospital.

Epstein had been placed on suicide watch after being found with bruising on his neck, but was taken off the watch at the end of July.

READ MORE: Epstein found dead in prison in apparent suicide as he awaited trial

Attorney General William Barr called for an investigation by the FBI and the Justice Department’s inspector general’s office.

“Mr Epstein’s death raises serious questions that must be answered,” Barr said.

The financier, 66, had been denied bail and faced up to 45 years behind bars on sex trafficking and conspiracy charges. He had pleaded not guilty and was awaiting trial.

Epstein’s death raises questions about how the Bureau of Prisons ensures the welfare of high-profile inmates. In October, Boston gangster James “Whitey” Bulger was killed in a federal prison in West Virginia where had just been transferred.

Senator Ben Sasse, a Republican member of the Senate Judiciary Committee, wrote to Barr saying “heads must roll” after the incident.

“Every single person in the Justice Department – from your Main Justice headquarters staff all the way to the night-shift jailer – knew that this man was a suicide risk, and that his dark secrets couldn’t be allowed to die with him,” Sasse wrote.

On Friday, more than 2000 pages of documents were released related to a since-settled lawsuit against Epstein’s ex-girlfriend by Virginia Giuffre, one of Epstein’s accusers.

The records contained graphic allegations against Epstein, and the transcript of a 2016 deposition of Epstein in which he repeatedly refused to answer questions to avoid incriminating himself.