Attorney General William Barr has said that there were "serious irregularities" at the federal jail where Jeffrey Epstein took his own life as he awaited trial on charges he sexually abused underage girls.

The 66-year-old financier was found on Saturday morning in his cell at the chronically short-staffed Metropolitan Correctional Center, in a unit known for holding notorious prisoners under extremely tight security.

"I was appalled, and indeed the whole department was, and frankly angry to learn of the MCC's failure to adequately secure this prisoner," Barr said at a police conference in New Orleans on Monday.

"We are now learning of serious irregularities at this facility that are deeply concerning and demand a thorough investigation. The FBI and the office of inspector general are doing just that."

He added, "We will get to the bottom of what happened and there will be accountability."

The manner in which Epstein killed himself has not been announced. An autopsy was performed on Sunday, but New York City Chief Medical Examiner Dr Barbara Sampson said investigators were awaiting further information.

A private pathologist, Dr Michael Baden, observed the autopsy at the request of Epstein's lawyers. Baden was the city's chief medical examiner in the late 1970s and has been called as an expert witness in high-profile cases, including O J Simpson's 1994 murder trial.

Epstein had been placed on suicide watch after he was found in his cell a little over two weeks ago with bruises on his neck, a person familiar with the jail's operations told The Associated Press.

But he had been taken off the suicide watch at the end of July, said the person, who wasn't authorized to discuss jail operations publicly and spoke on condition of anonymity.

Inmates on suicide watch are subjected to 24 hours per day of "direct, continuous observation," according to U.S. Bureau of Prisons policy. They are also issued tear-resistant clothing and placed in cells that are stripped of furniture or fixtures they could use to take their own lives.

After being returned to the jail's special housing unit, Epstein was supposed to have been checked on by a guard every 30 minutes, but that precaution was not taken the night before his death, The New York Times reported on Sunday, citing an unidentified law enforcement official.

On the morning of Epstein's apparent suicide, guards on his unit were working overtime shifts to make up for staffing shortages, the person familiar with the matter told the AP.

The person said one guard was working a fifth straight day of overtime and another was working mandatory overtime.

Epstein's death cut short a prosecution that could have pulled back the curtain on his activities and his connections to celebrities and presidents, though Barr vowed on Monday that the case will continue "against anyone who was complicit with Epstein."

"Any co-conspirators should not rest easy. The victims deserve justice and they will get it," he said.

According to police reports obtained by the AP, Epstein had a team of recruiters and employees who lined up underage girls for him.

In a court filing on Monday, Epstein's accusers said that an agreement he negotiated with federal prosecutors in Florida over a decade ago to grant immunity to his possible accomplices should be thrown out now that he is dead.

Under that 2008 agreement, Epstein pleaded guilty to prostitution-related state charges and served 13 months behind bars.

At the time of his death, Epstein was being held without bail and faced up to 45 years in prison on federal sex trafficking and conspiracy charges unsealed last month.

Epstein's death is the latest black eye for the Bureau of Prisons, which was already was under fire over the October beating death of Boston gangster James "Whitey" Bulger at a federal prison in West Virginia.

Taken together, the deaths underscore "serious issues surrounding a lack of leadership" within the bureau, said Cameron Lindsay, a former warden who ran three federal lockups, including the Metropolitan Detention Center in Brooklyn.

A defense attorney for Epstein, Marc Fernich, also faulted jail officials, saying they "recklessly put Mr Epstein in harm's way" and failed to protect him.

Staffing shortages worsened by a partial government shutdown prompted inmates at the New York City jail to stage a hunger strike in January after they were denied family and lawyer visits.

Eight months later, the jail remains so short-staffed that the Bureau of Prisons is offering guards a USD 10,000 bonus to transfer there from other federal lockups.

In the wake of Epstein's suicide, union president Eric Young of the American Federation of Government Employees Council of Prison Locals said a Trump administration hiring freeze at the Bureau of Prisons has led to thousands of vacancies and created "dangerous conditions" for prison workers and inmates.

In a statement, Young said that teachers, clerical workers and other support staff are regularly used to fill in for guards, and many guards are regularly forced to work 70- and 80-hour weeks.

Meanwhile, the French government called on prosecutors to open an investigation into Epstein's links to France. US authorities have said Epstein had a home in Paris and used a fake Austrian passport to travel to France in the 1980s. The Paris prosecutor's office did not immediately comment.