The National Archives found documents clearly marked as classified, including at the “top secret” level, among the 15 boxes of papers and memento former President Donald Trump improperly took home from the White House, The Washington Post reports, citing two people familiar with the matter. Those documents are now being kept in secure storage by the Justice Department while officials determine the next step.

A “top secret” classification, according to the National Archives, applies to documents in which unauthorized disclosure “could be expected to cause exceptionally grave damage to the national security.” Even if the Justice Department doesn’t launch a criminal investigation into how such materials ended up at Trump’s not-secure club, former federal prosecutor Brandon Van Grack tells the Post, “the FBI would want and need to review the information and conduct an investigation to determine what occurred and whether any sources and methods were compromised.”

One key question for federal or congressional investigators is how highly classified information ended up in Trump’s Mar-a-Lago boxes.

One person familiar with the scramble to pack up Trump’s belongings suggested some of the documents Trump piled up in the White House residence may have inadvertently ended up at Mar-a-Lago. But multiple people close to the former president told the Post that “Trump was very secretive about the packing of boxes that were retrieved from Mar-a-Lago last month, and did not let other aides — including some of his most senior advisers — look at them.”

And “Trump has been loath to return the boxes of documents he took from the White House, despite repeated efforts by the National Archives to obtain them,” starting last summer, when archivists noticed some high-profile records were missing, The New York Times reports. Eventually, “officials at the National Archives threatened to send a letter to Congress or the Department of Justice if he continued to withhold the boxes,” and Trump started going through the files in December. 

Trump spokesman Taylor Budowich told the Post that “a normal and routine process is being weaponized by anonymous, politically motivated government sources to peddle Fake News,” and the National Archives could “credibly dispute this false reporting” but isn’t.

The “top secret” document report comes atop other new revelations about Trump’s habitual mishandling of presidential records, including frequently tearing up documents, possibly trying to flush printed paper down the toilet, and using personal cellphones that avoided White House call logs.