Former vice-president Mike Pence and President Joe biden have also reported finding classified documents in their possession but have voluntarily allowed FBI searches for any remaining material that should have been given to the National Archives. Mr Biden's documents date to his time as former president Barack Obama's vice-president.
Mr Trump resisted efforts to have documents in his possession returned, and the FBI in August conducted a court-approved search of Mr Trump's Florida resort.
A federal judge later directed Mr Trump's attorneys to look for any other classified material still in his possession, a task given to an outside firm, according to the Washington Post.
The US Department of Justice had asked a federal judge to hold Mr Trump's office in contempt of court for not fully complying with their initial subpoena last year to return all classified materials, the Post and ABC have reported.
US Special Counsel Jack Smith is investigating whether Mr Trump or his associates obstructed the Justice Department's probe into his retention of thousands of government records, about 300 of which were marked classified.
Mr Trump said empty folders found at his Florida estate “were merely inexpensive and very common folders with… ‘Presidential Reading,' ‘Confidential,' ‘Classified,' or other words stamped on the front cover.”
“I would put them in a pile and keep them as momentous. Nothing wrong with that,” wrote Mr Trump, who is running for president again in 2024.
It was not clear whether he meant to write “mementos,” meaning they were souvenirs.
Mr Smith's office declined to comment. The special counsel is also investigating efforts to overturn Mr Trump's 2020 election loss that culminated in the deadly Jan 6, 2021, attack on the US Capitol.