Although Donald Trump has called it a hoax, among the files the FBI seized from the former president's mansion in August was reportedly classified information describing a foreign state's nuclear capabilities.
This was reported by the Washington Post, citing people who are said to be familiar with the matter.
It was not reported which country it was.
Majid Sattar
Political correspondent for North America based in Washington.
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Some of the classified information seized at Mar-a-Lago is said to describe top-secret operations that even senior members of the National Security Council cannot see, the newspaper continued.
Only the President and selected cabinet members could authorize other government officials to learn details from it.
In order to have access to this information, the general security clearance is not sufficient.
Individuals must argue why they should be included in the circle of initiates.
Secret files of this kind not only have to be locked, but also guarded by security personnel.
Such files would have been in Trump's private estate.
The FBI has seized a total of more than 300 files Trump took with him to Florida after he left the White House: 184 that the National Archives reclaimed from Trump earlier this year;
38, which an attorney for the former president turned over to investigators in June, and more than 100, which were seized on August 8 after the FBI issued a search warrant.
While the Office of the National Intelligence Director is currently conducting a damage analysis, the Justice Department in Washington has had to put its criminal investigation on hold for the time being.
A federal judge in Florida on Monday granted Trump's request to appoint a special assessor to review the material for documents that may fall under attorney-client privilege or executive privilege.
By Friday, the parties to the dispute should submit a list of suggestions for the personnel.
Since the assessor will examine classified information, he must also have a security clearance himself.