The Democratic chairmen of three congressional committees conducting impeachment inquiries into the Trump administration warned Secretary of State Mike Pompeo on Tuesday against what they considered his "intimidating department witnesses" to protect himself and President Donald Trump.

The heads of the House Foreign Affairs, Intelligence, and Oversight and Reform committees included the accusation in a broader statement announcing that Pompeo would serve as a witness in the ongoing investigation. Their probe centers on whether Trump used his position for personal benefit, specifically in a July phone call with Ukrainian President Volodymyr Zelenskiy in which Trump pressed the newly elected leader to open up investigations against Joe Biden, the Democratic presidential frontrunner, and his son.

The contents of the call became public as a result of a yet-unidentified whistleblower who formally filed a complaint against Trump's actions leading up to and including the July 25 call – an action that has led to a cascade of events that could threaten Trump's presidency.

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The committee chairmen cited reports from Monday that Pompeo reportedly listened in on the call, and hinted at subsequent reports that the chief diplomat intends to block investigators' access to American diplomats.

"Secretary Pompeo was reportedly on the call when the president pressed Ukraine to smear his political opponent. If true, Secretary Pompeo is now a fact witness in the House impeachment inquiry," Rep. Eliot Engel of New York, Rep. Adam Schiff of California and Rep. Elijah Cummings of Maryland wrote in a joint statement. "He should immediately cease intimidating department witnesses in order to protect himself and the president."

Blocking diplomats' testimony before Congress amounts to a crime and "will constitute evidence of obstruction of the impeachment inquiry," the trio wrote. They added that Congress may view such an act as obstruction, and that "any withheld documents and testimony would reveal information that corroborates the whistleblower complaint."

Trump and his closest advisers, including Pompeo, have reportedly scrambled in recent days to contain the fallout of the president's interactions with foreign leaders. Trump on Monday said his administration is trying to learn the identity of the whistleblower, who is reportedly a CIA analyst detailed to the White House.

News outlets in recent days have reported that Trump has had similar interactions with other foreign leaders in which he pressured them to help him personally, including Australian Prime Minister Scott Morrison. In each case, staffers at the White House reportedly moved details of the calls to classified servers – a breach of normal protocol.