US law enforcement officials said 14 of the 15 boxes retrieved from former president Donald Trump in May contained classified documents, providing cause for the FBI's August search of his home, a newly public affidavit reveals.
The documents ordered unsealed by federal Judge Bruce Reinhart and made public on Friday show that the FBI believed Mr Trump was potentially housing confidential documents in unauthorised locations at his Mar-a-Lago resort.
"The government is conducting a criminal investigation concerning the improper removal and storage of classified information in unauthorised spaces, as well as the unlawful concealment or removal of government records," the FBI affidavit said.
Affidavits typically contain vital information about an investigation, as agents outline to a judge why they want to conduct a search and why they believe evidence of a potential crime is likely to be uncovered there. The documents are normally sealed during an investigation.
The National Archives had repeatedly requested the Trump team to return documents taken from the White House after his presidency concluded and received 15 boxes in May. A total of 184 classified and top secret documents were found "un-foldered, intermixed with other records" along with personal handwritten notes.
The FBI said 67 were marked confidential, 92 as secret and 25 as top secret, including some that featured sensitive information on "human intelligence" sources.
It led the agency to believe there were additional classified documents being held in "unauthorised" locations at Mar-a-Lago, and the FBI said there was "probable cause to believe that evidence of obstruction will be found".
An earlier search warrant said the US government is also investigating Mr Trump for potentially violating the Espionage Act, which prohibits gathering, transmitting or losing national defence information.
It also said potential obstruction of justice was being investigated.
Separate documents from the Justice Department also unsealed on Friday shared reasoning behind why much of the 32-page affidavit was heavily redacted.
It said redactions were made to "protect the safety and privacy of a significant number of civilian witnesses, in addition to law enforcement personnel".
The department also said: "Although the public is now aware that the government executed a search warrant at the premises owned by the former president and seized documents marked as classified, the affidavit is replete with further details that would provide a road map for anyone intent on obstructing the investigation."
US President Joe Biden was asked by a reporter on Friday if national security was threatened by documents at Mar-a-Lago.
"We’ll let the Justice Department determine that," Mr Biden said.
Records already made public by the Justice Department showed that the FBI had recovered 11 sets of classified documents, some listed as top secret, from Mr Trump's estate in the August 8 search.
One box included "SCI" or "sensitive compartmented information" that requires the material be protected in approved facilities and only viewed by those with the highest level of security clearance.
US media reported that Mr Trump kept at least 300 classified documents at his resort in Palm Beach, Florida.
Mr Trump has made numerous claims on his Truth Social media platform that he did nothing wrong by taking the documents from Washington and that he had declassified the documents.
He also has, without evidence, claimed the search was politically motivated.
Additional reporting by Kyle Fitzgerald