“We are now learning of serious irregularities at this facility that are deeply concerning and demand a thorough investigation,” Mr. Barr said. “There will be accountability.”

Mr. Epstein’s death came just two weeks after he was taken off suicide watch and was left unsupervised long enough to have apparently taken his own life, security lapses that have prompted a public outcry. And only one of the two people guarding Jeffrey Epstein normally worked as a correctional officer, according to three prison officials.

Business dealings: Lawyers, bankers and accountants have been trying to understand the sources of Mr. Epstein’s wealth and how he used it. Tens of millions of dollars flowed through his bank accounts, shell companies and, at times, charities.

Conspiracy theories: Shortly after his death, it seemed that anyone with 10 fingers and a Twitter account was providing, promoting or questioning a conspiracy theory. Among them: President Trump, Mayor Bill de Blasio of New York and MSNBC’s Joe Scarborough.

Perspective: Our reporter spoke to Mr. Epstein at his cavernous Manhattan home a year ago. The financier said, among other things, that sex with teenage girls should be acceptable, and that he had dirt on powerful people.