American financier Jeffrey Epstein had to be taken to hospital after he was found injured in his prison cell, US media has reported.
Mr Epstein is currently being held in a US federal jail where he is awaiting trial on sex trafficking charges.
A source said it was not clear whether the bruising on his neck was self-inflicted or from an assault.
He was treated for his injuries and remains in custody at the Metropolitan Correctional Centre in New York City.
Jail officials and Epstein's lawyers have not responded to the alleged assault.
Epstein was refused bail after a judge ruled he poses a danger to the public and was a possible flight risk.
The 66-year-old is accused of having sex with girls as young as 14-years-old in the early 2000s. He has pleaded not guilty.
The Metropolitan Correctional Centre, where Epstein is being held, is renowned for its high-profile prisoners. Among those include terrorists and until recently, Mexican drug lord Joaquin “El Chapo” Guzman.
Epstein pleaded guilty in Florida in 2008 to soliciting a minor for prostitution in a deal that ended a federal investigation involving many teenage girls who said they had been sexually abused.
He was sentenced to 13 months in jail but was allowed to leave the county stockade on most days and spend the day in his private office.
After his release, he registered as a sex offender.
US federal prosecutors in New York reopened the probe after investigative reports in the Miami Herald sparked outrage over the original plea bargain.
Epstein was indicted on new federal charges this month.
Epstein’s lawyers say he has not had any illicit contact with underage girls since serving his jail sentence in Florida.
They also say the current charges are improper because the federal government is reneging on the deal not to prosecute him.
US labour secretary Alexander Acosta resigned after coming under renewed criticism for overseeing that deal when he was US attorney in Miami.