The financier, who once counted Republican President Donald Trump and Democratic former President Bill Clinton as friends, was arrested on July 6 and pleaded not guilty to charges of sex trafficking involving dozens of underage girls as young as 14, from at least 2002 to 2005.

The FBI and the Department of Justice's Inspector General were opening investigations into his death, US Attorney General William Barr said, adding that he was "appalled" to learn of the apparent suicide in federal custody.

Last month, Epstein was found unconscious on the floor of his jail cell with marks on his neck, and officials were investigating that incident as a possible suicide or assault.

"Mr. Epstein’s death raises serious questions that must be answered," Barr said in a statement. Even though Epstein was found unconscious last month, he had recently been taken off suicide watch, a special set of procedures for inmates in danger of taking their own life, according to the source. 

The financier was in a cell by himself when his body was found. It was not immediately clear why Epstein was taken offsuicide watch. 

At the MCC, two jail guards are required to make separate checks on all prisoners every 30 minutes, but that procedure was not followed over night, according to the source. 

In addition,every 15 minutes guards are required to make another check on prisoners who are on suicide watch. 

Guards generally remove any prisoner placed on suicide watch from the Special Housing Unit where Epstein was housed, according to a Twitter post by Preet Bharara, a former Manhattan-based US attorney. 

The unit separates inmates suchas accused sex offenders from the general population for theirprotection. 

Aja Davis, a spokeswoman for the New York City Office of theChief Medical Examiner, said she could not say how Epstein died before her office examined the body.

Epstein was arrested on July 6 at a New Jersey airport after taking a private plane from Paris. Last month, a judge refused Epstein's request to release him on bail and allow him to live under 24-hour guard in his opulent townhouse on Manhattan's Upper East Side. The financier was appealing that ruling.

The indictment accused Epstein of knowingly recruiting underage women to engage in sex acts with him, sometimes over a period of years while paying the women for each encounter. 

Prosecutors said a search of his townhouse, conducted under a warrant, uncovered evidence of hundreds, and perhaps thousands, of nude photographs of “what appeared to be underage girls,” including some photos cataloged on compact discs and kept in a locked safe.

In a court filing on July 25, the government said it was pursuing an "ongoing investigation of uncharged individuals" in connection with the case. That investigation, by the US Attorney’s Office in Manhattan, will continue despite Epstein's death, a different source familiar with the matter said.

"Today’s events are disturbing, and we are deeply aware of their potential to present yet another hurdle to giving Epstein’s many victims their day in court," Geoffrey Berman, the US Attorney in Manhattan, said in a statement. 

"Our investigation of the conduct charged in the indictment which included a conspiracy count remains ongoing." In addition to Trump and Clinton, Epstein has socialized with L Brands founder Les Wexner over the years, and according to court papers, with Britain's Prince Andrew. 

None of them was mentioned in the indictment against Epstein. The charges were announced more than a decade after he pleaded guilty in Florida to state charges of solicitation of prostitution from a minor in a deal with prosecutors that has been widely criticized as too lenient.

One of the Florida prosecutors, Alexander Acosta, resigned as US labor secretary in July as the deal came under fresh scrutiny after Epstein's arrest in New York.

His death came a day after the unsealing of a court filing in which a woman who accused Epstein of keeping her as a sex slave said one of the financier's associates had instructed her to have sex with at least a half-dozen prominent men.

The claim by Virginia Giuffre came in a deposition that was included in about 2,000 pages of documents related to her defamation lawsuit against Ghislaine Maxwell, the associate whom Giuffre said helped Epstein procure girls for sex.

Lawyers for Maxwell did not respond to several phone andemail requests for comment. (Reuters)