The law-enforcement official with knowledge of the investigation said that when the decision was made to remove Mr. Epstein from suicide watch, the jail informed the Justice Department that Mr. Epstein would have a cellmate and that a guard “would look into his cell” every 30 minutes.

But that was apparently not done, the official said, speaking on the condition of anonymity because the death was still under investigation.

The city’s chief medical examiner, Dr. Barbara Sampson, announced on Sunday night that her office had conducted an autopsy of Mr. Epstein, but she declined to release a determination about the cause of death.

She said a private pathologist, Dr. Michael Baden, who was hired by Mr. Epstein’s lawyers, had observed the autopsy examination.

The medical examiner is confident the cause of death is suicide by hanging, but she wants more information from law enforcement before releasing her determination, a city official said.

[Read more: Why the investigation into the Epstein sex trafficking accusations is not over.]

Senior law-enforcement officials, members of Congress and Mr. Epstein’s accusers have all demanded answers about why Mr. Epstein was not being more closely monitored. On Sunday, the Bureau of Prisons offered no explanation for why Mr. Epstein was left alone and not checked on.

Mr. Epstein’s death has also unleashed a torrent of unfounded conspiracy theories online, with people suggesting, without evidence, that Mr. Epstein was killed to keep him from incriminating others.