Through a wave of new subpoenas and grand jury testimony, the Justice Department is moving aggressively to develop a fuller picture of how the documents Mr. Trump took with him from the White House were stored, who had access to them, how the security camera system at Mar-a-Lago works and what Mr. Trump told aides and his lawyers about what material he had and where it was, the people said.
At the heart of the inquiry is whether Mr. Trump sought to hide some documents after the Justice Department issued a subpoena last May demanding their return.
The existence of an insider witness, whose identity has not been disclosed, could be a significant step in the investigation, which is being overseen by Jack Smith, the special counsel appointed by Attorney General Merrick B. Garland.
The witness is said to have provided investigators with a picture of the storage room where the material had been held.
Little else is known about what prosecutors might have learned from the witness or when the witness first began to provide information to the prosecutors.