Former President Trump reportedly held on to more than 300 classified documents after leaving office, half of which were recovered in January by the National Archives, which alerted the Justice Department in what eventually led 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 material 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 in Florida, was given to the Justice Department in June by Trump aides. Additional classified documents were recovered in the FBI search earlier this month, totaling more than 300 such documents in all, according to the Times.

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