Some classified documents, like those found at former President Donald Trump's Mar-a-Lago home, are so sensitive that only a few people are allowed to view them.
Some of the documents seized by the FBI last month contained highly confidential information about a foreign government's nuclear capabilities.
According to The Post, documents about highly classified operations are supposed to be kept in a secure facility. An officer may be assigned to look after the documents.
More than 10,000 government documents were found when federal agents searched Trump's home. The Espionage Act is being investigated by the Department of Justice.
According to a former counterintelligence official, the investigations seem to be moving towards criminal charges for Trump.
A federal judge in Florida approved Trump's request for a third party to review the seized files.
Representatives for Trump, The White House, and the Office of the Director of National Intelligence didn't reply to Insider's request for comment.