The subpoena to Mr. Navarro — which he said the F.B.I. served at his house last week — seeks his testimony about materials related to the buildup to the Jan. 6 attack on the Capitol, and signals that the Justice Department investigation may be progressing to include activities of people in the White House.

Mr. Navarro revealed the existence of the subpoena in a draft of a lawsuit he said he is preparing to file against the House committee investigating the Jan. 6 attack, Speaker Nancy Pelosi and Matthew M. Graves, the U.S. attorney for the District of Columbia.

Mr. Navarro, who plans to represent himself in the suit, is hoping to persuade a federal judge to block the subpoena, which he calls the “fruit of the poisonous tree.”…

Mr. Navarro has refused to cooperate with the committee. He was found in contempt of Congress, and the House referred the contempt case to the Justice Department for possible criminal prosecution. In his draft lawsuit, he called the committee’s subpoena “illegal and unenforceable.”