At some point after National Archives officials realized they did not have Trump White House documents, which are required to be preserved under the Presidential Records Act, they contacted [Patrick F.] Philbin [deputy White House counsel] for help returning them.
...
Mr. Philbin tried to help the National Archives retrieve the material, two of the people familiar with the discussions said. But the former president repeatedly resisted entreaties from his advisers.

“It’s not theirs, it’s mine,” several advisers say Mr. Trump told them.

Not the first time he's been wrong but he was wrong.

...
...officials...used a subpoena to obtain surveillance footage of the hallway outside a storage room at Mar-a-Lago and saw something that alarmed them. They also received information from at least one witness who indicated that more material might remain at the residence, people familiar with the investigation said.