“I was appalled and frankly angry to learn of the [jail’s] failure to adequately secure this prisoner,” said  Barr, speaking Monday at a conference for police officers in New Orleans. “We are now learning of serious irregularities at this facility that are deeply concerning and demand a thorough investigation.”

AfterEpstein was removed from suicide watch, he was downgraded to “special observation status,” which mandated that guards check on him every 30 minutes and that he has a cellmate, a person familiar with his detention said.

But in the hours before Epstein’s death, his cellmate had left and wasn’t quickly replaced as required, the person said. The identity of Epstein’s cellmate couldn’t be determined. Epstein’s also wasn’t receiving the mandated regular check-ins, the person said.

“We will get to the bottom of what happened, and there will be accountability,” Barr said. The Federal Bureau of Investigation and the Justice Department’s inspector general have devoted additional manpower and resources to investigate Epstein’s death.

Lawmakers, meanwhile ratcheted up pressure on the department for answers. Bipartisan leaders of the House Judiciary Committee, which has oversight of the Justice Department, wrote in a letter Monday to Hugh Hurwitz, acting director of the Bureau of Prisons, demanding answers to a number of questions about officials’ handling of Epstein. Epstein’s death “demonstrates severe miscarriages of or deficiencies in inmate protocol,” the letter said.

Mr. Barr on Monday also promised the Justice Department would continue to investigate people who allegedly helped Mr. Epstein recruit young women and girls for sex trafficking.

“Any co-conspirators should not rest easy,” Barr said. “The victims deserve justice, and they will get it.”

The Manhattan-based federal prosecutors handling Epstein’s case also could seek to seize his assets through a process known as civil forfeiture. That process could allow prosecutors to sue for assets that were used to facilitate the alleged sex trafficking, including Epstein’s real estate, jets and bank accounts. Money from forfeited properties goes into a Justice Department fund that can be distributed to victims.

Through the civil forfeiture process, prosecutors could lay out the evidence they have gathered against Epstein that hasn’t become public, according to former federal prosecutors. The government could use investigative tools, such as search warrants and grand jury subpoenas, unavailable to victims suing the estate, said Jaimie Nawaday, a former federal prosecutor who has handled asset-forfeiture cases.

“It might be in their interest to let the government go first,” said  Nawaday, now a partner at Kelley Drye & Warren LLP. “The government has a higher rate of success.”

Epstein’s body was released after the medical examiner performed an autopsy Sunday. His only known living relative is his brother, Mark Epstein. It couldn’t be determined if Epstein had prepared a will, although estate-law experts said he likely had one given his wealth.

Lawyers for Epstein’s accusers on Monday told a federal judge in Florida that Epstein’s death should invalidate a deal he struck with Florida prosecutors in 2007, which includes a provision that immunizes any of his potential co-conspirators from being charged with federal sex crimes.

The request was made in a court filing as part of a lawsuit filed in 2008 against the government that challenged the deal on the grounds it violated victims’ rights under federal law.

Invalidating that provision would remove what is likely to be a key defence argument for any co-conspirators who are charged in New York—that they can’t face federal prosecution because it is barred by the 2007 deal. It would also allow federal prosecutors in Florida to pursue any individuals involved in the alleged scheme who didn’t commit any crimes in New York.

Regarding Epstein, federal prosecutors in Manhattan had argued they were allowed to charge him, in part because they had found new conduct in New York that wasn’t covered by the Florida deal.

In a sign that the investigation was continuing, federal agents searched Epstein’s property in the U.S. Virgin Islands on Monday, according to a person familiar with the matter. The move came more than a month after a search of  Epstein’s townhouse in Manhattan, during which authorities said they found hundreds of pornographic photos and a locked safe filled with cash and diamonds.

Epstein was arrested last month on two federal charges related to sex trafficking of minors, for which he faced a maximum sentence of 45 years in prison. He pleaded not guilty and had been awaiting trial at the detention facility after a federal judge denied his release on bail.

Prosecutors accused Epstein of orchestrating a yearslong sex-trafficking operation in which he and his associates lured dozens of girls—some as young as 14 years old—to his homes in New York and Florida to perform massages that involved sex acts.

Mr. Epstein’s death ratchets up the scrutiny of the Metropolitan Correctional Center, the facility in lower Manhattan where Epstein was housed. Inmates there have long complained about the conditions of confinement. After Epstein’s death, prison union officials said the jail suffers from staffing shortages that result in overworked guards.

“The institution is unsafe,” said Serene Gregg, the local union president that represents MCC workers. “Things are not adequately being performed.”

The Bureau of Prisons makes its own determinations about how inmates should be housed and monitored in detention facilities, according to law enforcement experts. Prosecutors have limited input into conditions of confinement and typically intervene only for safety reasons, such as if one inmate is cooperating against another and needs to be separated.

When Epstein was removed from suicide watch, the jail told Justice Department officials he would receive a cellmate and guards would look in on him every 30 minutes, a person familiar with the matter said.

The MCC houses nearly 800 inmates, most of whom are in pre-trial detention. Joaquín “El Chapo” Guzmán, the notorious drug-cartel leader who escaped twice from maximum-security prisons in Mexico, spent two years awaiting U.S. trial at MCC without any major incidents.

A handful of criminal prosecutions each year involves corruption among correctional officers at MCC. Last year, one officer pleaded guilty in Manhattan federal court to smuggling alcohol, cellphones and other contraband for inmates in exchange for more than $25,000 in bribes.