Millionaire financier Jeffrey Epstein had been taken off suicide watch before he apparently killed himself in jail in New York while awaiting trial on sex trafficking charges.
The US Bureau of Prisons said Epstein was found unresponsive in his cell in the Metropolitan Correctional Centre in Manhattan at about 6.30am local time on Saturday and was pronounced dead a short time later.
US Attorney General William Barr said he was "appalled" to learn of Epstein's death while in federal custody, and said the FBI and Department of Justice will both investigate in a bid to answer "serious questions" about what happened.
Epstein, 66, had been denied bail and faced up to 45 years behind bars on federal sex trafficking and conspiracy charges.
He had pleaded not guilty to accusations of sexually abusing dozens of underage girls.
Just over two weeks ago, Epstein was found on the floor of his cell with bruises on his neck.
At the time, it was not clear whether the injuries were self-inflicted or from an assault.
A source said Epstein had been taken off suicide watch before his death, although it is not clear when.
The Bureau of Prisons confirmed he had been in the jail's Special Housing Unit, a heavily secured part of the facility that separates high-profile inmates from the general population.
Until recently, the same unit had been home to the Mexican drug lord Joaquin "El Chapo" Guzman, who is now serving a life sentence at the so-called Supermax prison in Colorado.
Some of his accusers and their lawyers reacted with frustration to the news that the financier will not now have to face them in court.
"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," accuser Jennifer Araoz said in a statement.
Brad Edwards, a Florida lawyer for nearly two dozen other accusers, said it was "not the ending anyone was looking for".
He added: "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."
Epstein's arrest last month saw separate investigations launched into how authorities handled his case initially when similar charges were brought against him more than a decade ago.
In that case in 2008, a deal allowed Epstein to plead guilty to soliciting a minor for prostitution in Florida and avoid more serious federal charges.
Federal prosecutors in New York reopened the probe after investigative reporting by The Miami Herald stirred outrage over that plea bargain.
US Labour Secretary Alexander Acosta resigned last month after coming under fire for overseeing that deal when he was US attorney in Miami.
Epstein's lawyers maintained the new charges brought by federal prosecutors in New York were covered by the deal and were improper.
They said he has not had any illicit contact with underage girls since serving his 13-month sentence in Florida.
On Friday, more than 2,000 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 contain graphic allegations against Epstein, as well as the transcript of a 2016 deposition of Epstein in which he repeatedly refused to answer questions to avoid incriminating himself.
Sigrid McCawley, Ms Giuffre's lawyer, claimed Epstein's suicide less than 24 hours after the documents were unsealed "is no coincidence".
Ms McCawley urged authorities to continue their investigation, focusing on Epstein associates who she alleged "participated and facilitated Epstein's horrifying sex trafficking scheme".