Rudy Giuliani has been subpoenaed by the House Intelligence Committee to produce documents by Oct. 15 related to his and President Trump's alleged efforts to push the government of Ukraine to investigate Joe Biden.

Why it matters: Over the past year, Giuliani has met with a variety of Ukrainian officials in an effort to investigate baseless allegations that Biden forced Ukraine to fire a prosecutor investigating his son, Hunter. In a July phone call that is now at the heart of House Democrats' impeachment inquiry, Trump asked Ukraine's president to work with Giuliani to investigate Biden — prompting allegations by a whistleblower that Trump was soliciting foreign interference in the 2020 election.

The chairmen of the House Intelligence, Foreign Affairs and Oversight committees wrote in a letter that in addition to admitting on national television that he asked Ukraine to investigate Biden, Giuliani stated that he is in possession of evidence that suggests other Trump administration officials are involved in the scheme.

stated that he is in possession of evidence that suggests other Trump administration officials are involved in the scheme. Giuliani recently told the Washington Post that he has "about 40 texts" from State Department officials asking him to do what he did.

The chairmen also sent letters to 3 of Giuliani's business associates — Lev Parnas, Igor Fruman, Semyon “Sam” Kislin — seeking documents and scheduling depositions.

The big picture: The House Intelligence Committee is moving full steam ahead with its impeachment investigation, having also subpoenaed Secretary of State Mike Pompeo and scheduled a hearing with Intelligence Community Inspector General Michael Atkinson for Friday. The committee is also working to secure testimony from the anonymous whistleblower.

What to watch: Giuliani said on Sunday that he will cooperate with the committee's investigation if Trump asks him to, but attacked Rep. Adam Schiff (D-Calif.) as an "illegitimate chairman" who deserves to be removed.

Read the letter to Giuliani: