The chairmen of three House committees subpoenaed President Trump's personal attorney Rudy Giuliani on Monday for key documents related to the Ukraine controversy as part of their formal impeachment inquiry against Trump.

“Pursuant to the House of Representatives’ impeachment inquiry, we are hereby transmitting a subpoena that compels you to produce the documents set forth in the accompanying schedule by October 15, 2019,” Intelligence Committee Chairman Adam Schiff, Foreign Affairs Committee Chairman Eliot Engel and Oversight Committee Chairman Elijah Cummings, all Democrats, wrote.

Giuliani tweeted Monday night, “I have received a subpoena signed only by Democrat Chairs who have prejudged this case. It raises significant issues concerning legitimacy and constitutional and legal issues including,inter alia, attorney client and other privileges. It will be given appropriate consideration.”

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Giuliani, the former New York City mayor, played a key role in seeking information from Ukrainian officials on former Vice President Joe Biden's dealings with the country, along with those of his son Hunter.

The committee chairs subpoenaed Giuliani after claiming he admitted to being "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."

Fox News reported on Sunday that in addition to Giuliani, Washington, D.C., lawyers Joe diGenova, a former U.S. attorney, and his wife Victoria Toensing, worked alongside him. According to a top U.S. official, the three attorneys were working “off the books” — not within the Trump administration — and only Trump knew the details of their work.

“A growing public record indicates that the president, his agent Rudy Giuliani, and others appear to have pressed the Ukrainian government to pursue two politically-motivated investigations,” the chairmen wrote. “The committees have reason to believe that you have information and documents relevant to these matters.”

The subpoena for Giuliani came as the committees have investigated the president’s controversial phone call with Ukrainian President Volodymyr Zelensky in July. House Democrats' impeachment inquiry has quickly expanded since Speaker Nancy Pelosi, D-Calif., signed off on it last week, with committee leaders issuing subpoenas to Giuliani and key Trump administration officials in an effort to obtain testimony to bolster their case against the president.

Over the weekend, Schiff, D-Calif., Engel, D-N.Y., and Cummings, D-Md., subpoenaed Secretary of State Mike Pompeo for documents related to Ukraine they said were critical to their probe.

GIULIANI WAS NOT WORKING ALONE IN BIDEN-UKRAINE PROBE

Schiff has also scheduled closed-door testimony from intelligence community Inspector General Michael Atkinson, who is slated to appear before the panel on Friday. Atkinson is the intelligence community watchdog who first received the whistleblower complaint and transmitted it to Acting Director of National Intelligence Joseph Maguire.

Schiff has also scheduled depositions for several State Department officials later next month.

In their demands to the State Department, Schiff and the committee chairs asked for correspondence and documents dating back to Jan. 20, 2017. The documents related to revelations that Trump asked Ukrainian officials to investigate Joe and Hunter Biden, as well as any Biden business activity in Ukraine. They also demanded a list of State Department officials who may have been involved in the July 25 Trump-Zelensky phone call, as well as State Department records.

Further, the chairs notified Pompeo that they have scheduled depositions for State Department officials slated for next month. The depositions are for former Ambassador to Ukraine Marie Yovanovitch, former U.S. Special Representative for Ukraine Kurt Volker—who resigned from his post last Friday—Deputy Assistant Secretary George Kent, Counselor T. Ulrich Brechbuhl and U.S. Ambassador to the European Union Gordon Sondland.

All officials, except Kent, were named in the whistleblower complaint accusing Trump of “using the power of his office to solicit interference from a foreign country in the 2020 U.S. election.” The whistleblower alleged that Trump pressured the Ukrainian president into investigating the Bidens, and Democrats have claimed this appeared linked to U.S. military aid to Ukraine -- which Trump denied.

Also Monday, Senate Judiciary Committee Ranking Member Dianne Feinstein, D-Calif., and the panel's fellow Democrats wrote to Chairman Lindsey Graham, R-S.C., suggesting the newly revealed documents “raise additional concerns that the White House has known about and, potentially aided by top officials at the Department of Justice, may have taken steps to conceal presidential misconduct.” They urged Graham to pursue the matter and work to protect any witnesses from potential intimidation.

The phone call revealed that the president wanted to get Zelensky on the phone with Giuliani, as well as Attorney General Bill Barr, to discuss a potential investigation.

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Barr, who has sought to distance himself from the controversy, has not been subpoenaed, and it was unclear whether the committees will attempt to compel his testimony.

Democrats, though, have planned to continue working expeditiously on the inquiry, with some reports indicating they could even be prepared to introduce formal articles of impeachment against Trump later this fall.

Fox News' Ashley Cozzolino, Jason Donner and The Associated Press contributed to this report.