A week after being denied bail, Jeffrey E. Epstein was found unconscious in his cell on Tuesday at a federal jail in Manhattan with marks on his neck, and prison officials were investigating the incident as a possible suicide attempt, a law enforcement official who had been briefed on the matter said.

Prison officials had not ruled out the possibility, however, that Mr. Epstein had been assaulted by another inmate or had staged the incident, a person with knowledge of the investigation said.

Mr. Epstein’s injuries were not serious, the law enforcement official said. A second law enforcement official confirmed that Mr. Epstein had been discovered in his cell with “bruising around the neck.” All three people spoke on the condition of anonymity because they were not permitted to publicly discuss the investigation.

The Bureau of Prisons, in an email on Thursday morning, gave no details about the incident, citing “privacy and security reasons.” The bureau said Mr. Epstein, 66, was not in a hospital but still at the jail, the Metropolitan Correctional Center in Lower Manhattan, where he has been held on sex trafficking and conspiracy charges.