US financier Jeffrey Epstein has been found dead in his prison cell while awaiting trial on sex trafficking charges.
His body was discovered at 06:30 local time (10:30 GMT) on Saturday at a facility in New York.
Officials said his death was an apparent suicide. The FBI is investigating the incident.
Epstein, 66, pleaded not guilty to sex trafficking and conspiracy charges last month and was being held without bail.
"[He] was found unresponsive in his cell in the Special Housing Unit from an apparent suicide," a statement from the justice department said.
"Mr Epstein was transported... to a local hospital for treatment of life-threatening injuries, and subsequently pronounced dead," the statement added.
Last month, shortly after he was denied bail, Epstein was found semi-conscious in his cell with injuries to his neck. He was treated at a nearby hospital, reports said, before being returned to the Metropolitan Correctional Center in New York.
The facility is considered to be one of the most secure in the country.
There are conflicting reports as to whether Epstein was placed on suicide watch following that incident, which prison officials have been investigating as a possible suicide attempt.
But US media report that, despite the apparent warning signs, the financier was not on suicide watch at the time of his death.
Attorney General William Barr said there were "serious questions that must be answered". He added that he was "appalled" by the news and said the justice department had opened its own investigation.
Epstein's death comes a day after hundreds of pages of court documents were released that revealed new allegations against him and some of his associates.
What was he charged with?
Epstein was accused of paying girls under the age of 18 to perform sex acts at his Manhattan and Florida mansions between 2002 and 2005.
He was arrested on 6 July after landing in New Jersey on his private jet. He avoided similar charges in a controversial secret plea deal in 2008, and instead pleaded guilty to a lesser charge.
That plea deal was closely scrutinised in recent weeks and, last month, US Labor Secretary Alex Acosta resigned over his role in it.
Prosecutors also accused Epstein of paying large amounts of money to two potential witnesses ahead of his trial, which was scheduled to take place next year.
He faced up to 45 years in prison if convicted.
What reaction has there been?
Politicians and other senior officials have reacted with shock, asking how Epstein's death was allowed to happen given his high-profile status and the apparent suicide attempt in July.
Michael Bromwich, a former inspector general at the justice department, called for an "immediate and comprehensive... investigation to determine who is responsible".
One union leader for federal prison workers, Joe Rojas, told NBC News that Epstein "should have been on suicide watch".
"You don't have to be a psychologist. It's common sense," he told the network on Saturday.
Elsewhere, Epstein's alleged victims have expressed disappointment that he will no longer stand trial. "I am extremely mad and hurt thinking he once again thought he was above us and took the easy way out," Jena-Lisa Jones said in a statement.
"I am angry Jeffrey Epstein won't have to face the survivors of his abuse in court," another alleged victim, Jennifer Araoz, told CNBC.
"We have to live with the scars of his actions for the rest of our lives."
A lawyer for some of the alleged victims, Lisa Bloom, said she would still seek compensation for them.
"We would have preferred he lived to face justice. Our civil cases can still proceed against his estate. Victims deserve to be made whole for the lifelong damage he caused," she wrote on Twitter.
"I've sat with my clients as they have cried and talked about how their lives were changed forever," she told MSNBC. "They deserve compensation."
US Attorney for the Southern District of New York, Geoffrey Berman, said the news of Epstein's death was "disturbing".
"We are deeply aware of their potential to present yet another hurdle to giving Epstein's many victims their day in court," he said in a statement. "We remain committed to standing for you, and our investigation... remains ongoing."
Who was Jeffrey Epstein?
New York-born Epstein worked as a teacher before moving into finance.
Prior to the criminal cases against him, he was best known for his wealth and high-profile connections.
He was often seen socialising with the rich and powerful, including President Donald Trump, former President Bill Clinton and the UK's Prince Andrew.
In a 2002 profile in New York Magazine, Mr Trump referred to Epstein as a "terrific guy".
But he later said the pair fell out "12 or 15 years ago" and reiterated last month that he was "not a fan of Jeffrey Epstein".
Reports of Epstein's wealth vary, with his Virgin Islands-based firm generating no public records.
History of the allegations
2002: The earliest allegations of abuse covered by the recent case take place.
October 2002: Donald Trump tells New York magazine he has known Epstein for 15 years, and that he is a "terrific guy.... it is even said that he likes beautiful women as much as I do, and many of them are on the younger side".
2005: One of Epstein's alleged victims, aged 14, reports him to the police in Palm Beach - sparking the first investigation.
May 2006: Epstein is charged with unlawful sex acts with a minor. Later in the year, the case is referred to the FBI.
2007: A plea deal is struck with Alex Acosta, the US attorney in Florida. Instead of facing federal sex-trafficking charges, he pleads guilty to two charges of soliciting prostitution, including with a minor.
June 2008: Epstein is sentenced to 13 months in prison - a private wing of a county jail. He is also allowed to leave for work - up to 12 hours a day, six days a week. He does, however, have to register as a sex offender.
April 2017: Alex Acosta is appointed labor secretary by now-President Donald Trump.
November 2018: The Miami Herald publishes its explosive investigation into Epstein, the plea deal, and the dozens of women alleging abuse.