Investigations have begun into how disgraced financier Jeffrey Epstein took his own life in prison, ahead of facing trial on sex trafficking charges.

The 66-year-old, who was being held without bail in a Manhattan jail, was accused of abusing dozens of underage girls as young as 14 in New York and Florida.

Epstein, who denied the charges, was found at 6.30am on Saturday morning, according to a statement from the US Department of Justice.

US Attorney-General William Barr, in announcing the investigation by the FBI and the Justice Department's inspector general's office, said he was "appalled" to learn of Epstein's death while in federal custody.

"Mr Epstein’s death raises serious questions that must be answered.

"In addition to the FBI’s investigation, I have consulted with the Inspector General who is opening an investigation into the circumstances of Mr Epstein’s death", he said in a statement.

Statement from Attorney General William P. Barr on the Death of Jeffrey Epstein https://t.co/IdTt0sEfGj

— Justice Department (@TheJusticeDept) August 10, 2019

The fire department said it received a call at 6.39am that Epstein was in cardiac arrest and he was pronounced dead at New York-Presbyterian Lower Manhattan Hospital.

The statement from the Department of Justice said Epstein was found unresponsive in his cell from an apparent suicide, adding that life-saving measures were immediately initiated by responding staff.

Emergency medical services were requested and Epstein was transferred to hospital for treatment, before being pronounced dead.

The NYC Medical Examiner's vehicle in front of the Metropolitan Correctional Centre, where Jeffrey Epstein was found dead in his cell | Image: Vanessa Carvalho/Zuma Press/PA Images

NBC News is reporting that, contrary to earlier reports, Epstein was not on suicide watch at the time he was found.

He was facing 45 years in prison if convicted.

Last month, Epstein had reportedly been found unconscious with neck injuries on the floor of his cell. His lawyers declined to comment at the time.

A source said he had recently been taken off suicide watch, but it is not clear when.

According to the US Bureau of Prisons, he had been in the jail's special housing unit - a heavily secured part of the prison, which separates high-profile inmates from the general population.

It was the same unit where drug lord Joaquin "El Chapo" Guzman was held until he moved to the Supermax prison in Colorado.

Epstein pleaded guilty in 2008 to prostitution-related charges involving underage girls, in exchange for the Justice Department's agreement not to prosecute him on similar charges.

He served 13 months in jail and agreed to register as a sex offender under the 2008 plea deal, which is now widely seen as too lenient.

Federal prosecutors in New York had re-opened the case after reports from the Miami Herald stirred outrage.

His lawyers had argued the agreement barred federal prosecutors in Manhattan from pursuing the latest case over the same conduct.

However, US Attorney Geoffrey Berman said there was no such block.

This March 2017 file photo, provided by the New York State Sex Offender Registry, shows Jeffrey Epstein | Image: AP/Press Association Images

Alexander Acosta, the US Labour Secretary, resigned last month after criticism for overseeing the deal when he had been US attorney in Miami.

Accuser Jennifer Araoz said: "We have to live with the scars of his actions for the rest of our lives, while he will never face the consequences of the crimes he committed, the pain and trauma he caused so many people."

Brad Edwards, a lawyer for near two dozen other accusers, said it was "not the ending anyone was looking for".

He said: "The victims deserved to see Epstein held accountable, and he owed it to everyone he hurt to accept responsibility for all of the pain he caused."

On Friday, more than 2,000 pages of a settled lawsuit against Epstein's ex-girlfriend were released by accuser Virginia Giuffre.

They include graphic allegations against Epstein as well as the transcript of a 2016 deposition in which he refused to answer any questions to avoid incriminating himself.

Epstein also had links with Donald Trump. Video of them together at a party in 1992 emerged in June, and they were also pictured together five years later.

There is no suggestion that there was any criminal wrongdoing taking place at the party and Mr Trump now claims he never liked Epstein.

Main image: In front of the Metropolitan Correctional Centre, where Jeffrey Epstein was found dead in his cell at the Manhattan jail in New York City | Image: Vanessa Carvalho/Zuma Press/PA Images