The chairs of three House committees subpoenaed Rudy Giuliani on Monday, as part of the impeachment inquiry into President Donald Trump launched last week.

The subpoena requires Giuliani, Trumps personal lawyer and a former mayor of New York, to produce documents that concern Trump pressuring the president of Ukraine to investigate one of his political rivals, former Vice President Joe Biden.

The subpoena letter is signed by Democratic Reps. Adam Schiff (Calif.), chair of the House Intelligence Committee; Eliot Engel (N.Y.), chair of the House Foreign Affairs Committee; and Elijah Cummings (Md.), who chairs the House Oversight and Reform Committee.

In the letter, the three lawmakers note that Giuliani admitted on CNN that he asked the Ukrainian government to dig up dirt on Biden, whom Trump could face off against in the 2020 presidential election.

In addition to this stark admission, you stated more recently that you are in possession of evidence in the form of text messages, phone records, and other communications indicating that you were not acting alone and that other Trump Administration officials may have been involved in this scheme, the lawmakers wrote.

Giuliani must comply with the subpoena by Oct. 15. He quickly questioned its legality, while also saying in a tweet he will give it appropriate consideration.

Among the required documents are any Giuliani has that are related or refer to Bidens son Hunter, or to Burisma, the Ukrainian energy company where Hunter once served on the board. Trump has repeatedly pushed the conspiracy theory that Biden once used his power as vice president to help the company by pushing for the firing of the Ukrainian prosecutor investigating Burisma. No evidence has surfaced supporting that theory.

The subpoena also calls for any documents or communications that concern U.S. foreign assistance to Ukraine. According to a summary of Trumps July phone call with Ukrainian President Volodymyr Zelensky, Trump implied that such aid was at stake if the country did not investigate Biden.

Shortly after Schiff, Engel and Cummings announced their subpoena of Giuliani, Trump tweeted a video clip in which he defended the call with Zelensky, saying it was perfect and claiming that the whistleblower an intelligence official who filed a formal complaint about Trumps Ukraine efforts, and whose identity has not been made public is at fault for making a false report.

This story has been updated with comment from Rudy Giuliani.