FBI search of Mar-a-Lago helps show how probe of Trump documents has changed - The Washington Post

"One person familiar with the investigation said agents were conducting a court-authorized search as part of a long-running examination into why documents — some of them top-secret — were taken to the former president's private club and residence instead of shipped to the National Archives and Records Administration when Trump left office. The Presidential Records Act requires the preservation of memos, letters, notes, emails, faxes and other written communications related to a president's official duties." 

 And, this article states, earlier, boxes of top secret documents — you know, like those involving National security, were found to be stored in an unlocked basement of his resort, which employs and hosts hundreds of foreign nationals.

https://www.washingtonpost.com/national-security/2022/08/09/trump-fbi-search-mar-a-lago/