The US National Archives discovered more than 700 pages of classified documents at former president Donald Trump's Florida home, in addition to material seized this month by FBI agents.
This update comes from a May letter that the records agency sent to the Republican former president's attorney that was made public by an ally of Mr Trump.
The large quantity of classified material in 15 boxes recovered in January by the National Archives and Records Administration, some marked as "top secret," provides more insight into what led to the FBI's court-authorised 8 August search of Mr Trump's residence at the Mar-a-Lago resort in Palm Beach.
The agency is responsible for preserving government records.
The 10 May letter was sent to Mr Trump's attorney Evan Corcoran by Acting US Archivist Debra Steidel Wall.
It was released late yesterday by John Solomon, a conservative journalist who Mr Trump authorised in June to access his presidential records.
"Among the materials in the boxes are over 100 documents with classification markings, comprising more than 700 pages. Some include the highest levels of classification, including Special Access Program (SAP) materials," Ms Wall's letter said, referring to security protocols reserved for some of the country's most closely held secrets.
A National Archives spokesperson did not respond to requests for comment.
The letter contains additional information about Mr Trump's handling of classified materials and his efforts to delay federal officials from being able to review the documents.
The letter shows that Mr Trump's legal team repeatedly tried to stall the archives from letting the FBI and intelligence officials review the materials, saying that he needed more time to determine if any of the records were covered by a doctrine called executive privilege that enables a president to shield some records.
President Joe Biden's administration - specifically the Justice Department's Office of Legal Counsel - has determined that the materials were not covered by executive privilege.
It found that "there is no precedent" for a former president to shield records from a sitting president using executive privilege when the materials in question legally belong to the federal government, according to the letter.
Even after Mr Trump returned the 15 boxes to the archives, the Justice Department still suspected he had more classified material at Mar-a-Lago.
The 8 August search was part of a federal investigation into whether Mr Trump illegally removed documents from the White House when he left office in January 2021 after his failed 2020 re-election bid and whether he tried to obstruct the government's investigation into the removal of the records.
On 3 June, the department's head of counterintelligence and three FBI agents visited Mar-a-Lago to inspect a storage room and collect additional records. Mr Trump received a second subpoena later that month seeking surveillance footage from security cameras, which he also provided.
During the 8 August search, FBI agents recovered more than 20 additional boxes, containing about 11 sets of records marked as classified.
