(The Hill) — Former President Donald Trump held onto more than 300 classified documents since he left office, half of which were recovered in January by the National Archives, who alerted the Justice Department in what eventually led up to the FBI search of his Mar-a-Lago property.

The New York Times reported Monday, citing multiple sources who have been briefed on the matter, that the sheer volume of classified-marked materials recovered by the government is what triggered a federal criminal investigation into the former president.

The National Archives in January recovered 150 classified documents while another set, which was also at Mar-a-Lago, was given to the Justice Department in June by Trump aides. Additional classified documents were later recovered in the FBI search earlier this month, totaling 300 such documents in all, according to the Times.

Some of the 15 boxes turned over to the Archives earlier this year included CIA, FBI and National Security Agency documents involving national security, according to a person briefed, the Times reported.

Two days after the FBI searched Mar-a-Lago, the Washington Post reported that some of the documents recovered in that search were related to nuclear weapons.

The Times also reported Monday that authorities are seeking surveillance video of Mar-a-Lago leading up to the Aug. 8 search in an effort to find out how some of the documents were being handled there.

An unsealed warrant executed at Mar-a-Lago made public days after the search revealed that federal authorities are investigating Trump on possible violations of the Espionage Act, among other statutes. A property receipt of items seized by the FBI shows authorities recovered a trove of documents, including 11 sets of classified documents, some of which were marked top secret. 

Trump has insisted he declassified the documents obtained by the FBI but it is possible he did so without following the proper protocols. 

Trump and the National Archives have been at odds since he left office in 2021 after the agency determined Trump was in possession of White House documents that should have been turned over to the government when he ended his term.