A lawyer for President Donald Trump said Thursday that the White House had provided more than 20,000 pages of documents to special counsel Robert Mueller as part of the investigation into Russian interference in the 2016 U.S. election, which he said showed “unprecedented” cooperation by the administration.

John Dowd, who heads the president’s private legal team, said the White House’s document production included more than 5,000 pages related to former national security adviser Mike Flynn and Russia, and nearly 8,000 pages related to former FBI Director James Comey.

As part of his investigation, Mr. Mueller is examining whether Trump associates colluded in Moscow’s interference efforts, and whether the president obstructed justice in firing Mr. Comey while his agency’s Russia probe was under way. Mr. Trump has denied collusion, and Moscow has said it didn’t meddle in the election.

The documents were provided to the special counsel voluntarily, Mr. Dowd said in a statement. Mr. Mueller’s team has also interviewed more than 20 White House officials, including eight members of the White House Counsel’s Office, Mr. Dowd said.

Mr. Dowd called the White House’s document production “the most transparent response in history by a president to special counsel inquiries.”