The FBI raid on Donald Trump's Mar-a-Lago estate was partly based on suspicions of violations of the Espionage Act, according to a search warrant.
It emerged that, in the raid on Monday, agents recovered "top secret" documents, including some considered so sensitive they were only meant to be viewed in special government facilities.
On Friday night a judge publicly released the search warrant, which showed the FBI had been authorised to look for evidence of breaches of the Espionage Act, which prohibits the possession or transmission of national defence information.
The warrant also showed the FBI was authorsied to seize any evidence of obstruction, or destruction of documents.
Violations of the Espionage Act can lead to heavy prison sentences.
Eleven sets of classified documents were found at the former president's Florida resort, according to publicly released court documents.
An inventory compiled by FBI agents showed one set was marked only for viewing in specific government sites, four were marked "top secret", three "secret", and three "classified".
Another batch of material relating to the "President of France" was also discovered.
Photographs and a "handwritten note" were also among the items seized.
The warrant described Mar-a-Lago as "a mansion with approximately 58 bedrooms, 33 bathrooms, on a 17-acre estate."
It said the specific locations to be searched included the "45 Office" - Mr Trump's current office - and "all storage rooms, and all other rooms or areas within the premises used or available to be used by FPOTUS [the former president] and his staff."
It authorised the seizure of "any physical documents with classification markings".
Also to be seized were "communications in any form, regarding the retrieval, storage, or transmission of national defense information or classified material."
Mr Trump's lawyers argue he declassified the material taken to Mar-a-Lago before he left the White House, but it was unclear whether he had the authority to do so with the most sensitive documents.
The Justice Department was reported to have been concerned that, if such items were being stored at Mr Trump's home, they could become a target for foreign intelligence agencies.
Mr Trump denied reports that some of the documents could have involved nuclear programmes.
In a statement he said: "Nuclear weapons issue is a Hoax, just like Russia, Russia, Russia was a Hoax, two Impeachments were a Hoax, the Mueller investigation was a Hoax, and much more.
"Same sleazy people involved. Why wouldn’t the FBI allow the inspection of areas at Mar-a-Lago with our lawyer’s, or others, present. Made them wait outside in the heat, wouldn’t let them get even close - said 'ABSOLUTELY NOT.' Planting information anyone?"
An informant at Mar-a-Lago was believed to have tipped off the FBI before the search.
Attorney General Merrick Garland, the top US law enforcement officer, and an appointee of President Joe Biden, said he had personally approved the search.
Republicans have rallied around Mr Trump, claiming the raid was politically motivated.
Ari Fleischer, former press secretary to President George W Bush, said Mr Trump would be able to defend himself by saying he didn't know the documents were there.
He said: "If Donald Trump didn't himself pack up those boxes then it's very hard to see culpability for him.
"I have it on reliable authority Donald Trump himself never opened up those boxes, and has no idea what's in them. Do they think he's hunting around his own basement reading files?"
In a statement, Mr Trump said: "Number one, it was all declassified. Number two, they didn’t need to ‘seize’ anything. They could have had it anytime they wanted without playing politics and breaking into Mar-a-Lago.
"It was in secured storage, with an additional lock put on as per their request. They could have had it anytime they wanted, and that includes LONG ago."