The Department of Justice says the FBI is investigating Epstein's suicide.
The New York City medical examiner's office hasn't determined his manner of death.
Epstein, 66, was set to stand trial next year for allegedly sex trafficking dozens of girls at his home in NY and Florida.
But a cache of previously sealed legal documents, released on Friday by a federal appeals court, provides new, disturbing details about what was going on inside Mr. Epstein's homes and how his associates recruited young women and girls, including from a Florida high school.
The New York Times reported Epstein had hung himself and that his body was found Saturday morning around 7:30 a.m. NBC News reported that after Epstein was found, he was taken to a hospital in Lower Manhattan and arrived in cardiac arrest.
The attorney general said he has consulted with the inspector general who will be launching an investigation into the circumstances surrounding Epstein's death. "Mr. Epstein's death raises serious questions that must be answered", Barr said in a statement following the news of the death.
Attorney Jack Scarola represents some of Epstein's victims.
The Justice Department and the federal Bureau of Prisons did not immediately respond to requests for comment on Saturday.
It also comes just over two weeks after the 66-year-old was found unconscious in his cell with marks on his neck after an apparent suicide attempt.
"I'm dumbfounded. It's possible Epstein was placed on suicide watch for some time and then returned to the SHU, prematurely".
Epstein was asked in the videotaped deposition whether it was standard operating procedure for his former girlfriend to bring underage girls to him to sexually abuse.
The multimillionaire financier was being held without bail pending trial on child sex-trafficking charges.
The new charges renewed attentionon the plea deal, spurring outrage against it and leading to the resignation of Trump administration Labor Secretary Alex Acosta, who negotiated the plea agreement when he was the USA attorney in Miami. Clinton and Trump both said they hadn't seen Epstein in years and knew nothing of his alleged misconduct when new charges were brought against him last month.
"(Epstein's) estate, you can imagine, would be sued by a lot of potential victims.
"The reckoning of accountability begun by the voices of courageous and truthful victims should not end with Jeffrey Epstein's cowardly and shameful suicide", McCawley said in a statement.
Epstein's case had attracted widespread attention - in part because of his wealth and political connections, and in part because of a lenient plea deal he reached more than a decade ago to resolve similar allegations.