Jeffrey Epstein, the millionaire financier who is being held on federal sex trafficking charges, was found injured and in a fetal position in his cell at a New York City jail, sources close to the investigation told NBC News on Wednesday night.

Epstein, 66, was found semiconscious with marks on his neck in his cell at the Metropolitan Correctional Center in Manhattan some time in the last two days, the sources said. Epstein is on suicide watch, two sources said.

While the two sources said that Epstein may have tried to hang himself, a third source cautioned that the injuries weren't serious, questioning whether Epstein might have staged an attack or a suicide attempt to get a transfer to another facility.

Another source said that an assault hadn't been ruled out and that another inmate in Epstein's unit, identified by sources as Nicholas Tartaglione, had been questioned. Sources also said Epstein and Tartaglione were cell mates.

Tartaglione, 49, is a former police officer in Briarcliff Manor, New York, who was arrested in December 2016 and accused of killing four men in an alleged cocaine distribution conspiracy, then burying their bodies in his yard in Otisville, according to court records. He was charged with drug conspiracy and four counts of murder and is awaiting trial.

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Sources said Tartaglione claimed not to have seen anything and not to have touched Epstein.

An attorney for Tartaglione denied that his client attacked Epstein, saying that Tartaglione and Epstein got along well and that Epstein appeared to be fine Wednesday.

Attorneys for Epstein didn't immediately return calls for comment. A spokesperson for the Bureau of Prisons didn't return calls, and spokespersons for the U.S. Marshals and U.S. attorney's office declined to comment.

Epstein was arrested July 6 in Teterboro, New Jersey, as he returned from Paris on a private jet. He has pleaded not guilty, and a federal judge denied bail last week.

Epstein, whom President Donald Trump called a "terrific guy" in a 2002 interview with New York magazine, is registered as a sex offender in Florida. Former President Bill Clinton has flown on one of Epstein's planes on several occasions, according to flight records reviewed by NBC News.

Epstein pleaded guilty in 2008 to procuring a person younger than 18 for prostitution and felony solicitation of prostitution. He served a 13-month sentence in a Florida county jail and was granted a federal non-prosecution agreement by the office of then-U.S. attorney for Miami, Alex Acosta, who resigned as U.S. labor secretary this month as criticism of the plea agreement grew ever louder.

The non-prosecution agreement, which limited the scope of the deal to cover only the Miami area, would no longer protect Epstein if he committed illegal acts in other parts of the country.

Epstein is charged in a new indictment handed up in New York with one count of sex trafficking conspiracy and one count of sex trafficking. He could be sentenced up to 45 years in prison if he's convicted.

Federal prosecutors allege that from at least 2002 through 2005, Epstein paid girls as young as 14 hundreds of dollars in cash for sex at either his Manhattan townhouse or his estate in Palm Beach, Florida. Prosecutors said the acts began as massages, but turned sexual, and that he would pay his victims to recruit additional girls.

Palm Beach Sheriff Ric Bradshaw ordered an internal investigation be opened on Thursday to determine whether deputies assigned to monitor Epstein during his work release program violated any agency rules and regulations.

In February, the Justice Department opened an investigation into how federal lawyers handled Epstein's case after Sen. Ben Sasse, R-Neb., wrote a series of letters to the department demanding to know why the government cut what he called a "sweetheart deal" with Epstein.