The letter was signed by Attorney-General Pamela Bondi, Deputy Attorney-General Todd Blanche and Jay Clayton, the US Attorney in Manhattan.
A new law passed in November mandated that the entire trove of Epstein files be released by December 19, 2025.
Justice officials said late last month that they hope to release the rest of the documents by January 20.
Members of Congress who pushed the legislation say that the department has not released key documents they want to see.
“DOJ’s refusal to follow the law I passed in Congress and release the full files is an obstruction of justice,” Representative Ro Khanna (Democrat-California), one of the new law’s main sponsors, said in a statement.
“They also need to release the FBI witness interviews which name other men, so the public can know who was involved,” he said.
More than 400 lawyers and 100 specially trained document analysts “will dedicate all or a substantial portion of their workday” to getting documents ready for release, the officials told the judge.
The letter - a progress report of sorts - gives a glimpse into the daunting labour that lies ahead for federal officials.
Those reviewing the unreleased documents must determine whether each document falls under the law’s broad mandate, review the documents to redact information that could identify victims, and respond to requests from victims or their family members for additional redactions, according to the letter.
Officials offered similar explanations for a delay in releasing all unclassified Epstein documents last month, after the department failed to meet its deadline.
Epstein was arrested on federal sex-trafficking charges in 2019 and died in federal custody later that year. His death was ruled a suicide.
Judges and lawmakers say that over decades, he abused, trafficked and molested scores of girls, many of whom have come forward in court and in other public forums.
Epstein’s friendships with prominent political, business and cultural figures, including US President Donald Trump, also continue to be under intense scrutiny.
Trump had a long-standing friendship with Epstein. He has said he knew Epstein socially in Palm Beach, Florida, and that they had a falling out in the mid-2000s.
Trump has attributed the end of their relationship to a quarrel over a real estate deal and to Epstein hiring employees away from Trump’s Mar-a-Lago Club.
White House spokeswoman Karoline Leavitt has said Epstein was ejected from the club “for being a creep” to female workers there.
Trump has not been accused of participating in Epstein’s criminal conduct.
Documents released last month confirmed that the FBI received a complaint about Epstein as far back as 1996.
Epstein did not appear to come under serious law enforcement scrutiny until about a decade later, when he was arrested in 2006.
At the time, Epstein reached an agreement with officials in Florida that enabled him to plead guilty in 2008 to two state charges of soliciting prostitution, including one involving a minor, while avoiding federal charges and serving just over a year behind bars - with ample work-release privileges.
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