The U.S. Justice Department on Friday disclosed that it was investigating former President Donald Trump for removing White House records because it believed he illegally held documents including some involving intelligence-gathering and clandestine human sources - among America's most closely held secrets.

The department released a heavily redacted affidavit that underpinned the FBI's extraordinary Aug. 8 search of Trump's Florida residence in which agents seized 11 sets of classified records including some labeled "top secret" as documents that could gravely threaten national security if exposed.

In the affidavit, an unidentified FBI agent said the agency reviewed and identified 184 documents "bearing classification markings" containing "national defense information" after Trump in January returned 15 boxes of government records sought by the U.S. National Archives. Other records in those boxes, according to the affidavit, bore handwritten notes by Trump.

The search was part of a federal investigation into whether Trump illegally removed and kept documents when he left office in January 2021 after losing the 2020 election to President Joe Biden and whether Trump tried to obstruct the probe.

Trump, a Republican who is considering another presidential run in 2024, has described the court-approved search at the Mar-a-Lago estate in Palm Beach as politically motivated, and on Friday again described it as a "break-in."

Documents released with the affidavit revealed that "a significant number of civilian witnesses" knowledgeable about Trump's actions after leaving office were helping the probe, a rare disclosure.