The House Judiciary Committee has served former White House counsel Don McGahn with a subpoena as part of its investigation into obstruction of justice, a move to bring in a key player with direct knowledge about President Donald Trump’s efforts to undermine the Russia probe, according to a source familiar with the discussion.

The Democratic-led panel authorized subpoenas earlier this month for McGahn and four other White House officials as part of its sweeping investigation into possible obstruction of justice, corruption and abuse of power. But it waited until Monday to issue the subpoena to McGahn, seeking documents and testimony from the former White House counsel.

The move is the clearest indication yet that Nadler’s committee is plotting an extensive examination into the President’s actions outlined in the Mueller report. The committee has already served the Justice Department with a subpoena for the full Mueller report and the underlying evidence, demanding the documents by May 1.