The search appears to be focused on materials Trump took from the White House.

The search, according to multiple people familiar with the investigation, appeared to be focused on material that Mr. Trump had brought with him to Mar-a-Lago, his private club and residence, when he left the White House. Those boxes contained many pages of classified documents. Mr. Trump delayed returning 15 boxes of material requested by officials with the National Archives for many months, only doing so when there became a threat of action to retrieve them.......

Mr. Trump, who campaigned for president in 2016 criticizing Hillary Clinton’s practice of maintaining a private email server for government-related messages while she was secretary of state, was known throughout his term to rip up official material that was intended to be held for presidential archives. One person familiar with his habits said that included classified material that was shredded in his bedroom and elsewhere.