The White House has informed the Democratic-controlled House of Representatives that the administration of President Donald Trump won’t participate in the legislative chamber’s impeachment probe.
The letter on October 8 called the inquiry into Trump, a Republican, "illegitimate" labeling the proceedings as "baseless" and "unconstitutional."
It was the White House's most combative response yet to the investigation.
House Speaker Nancy Pelosi (Democrat-California) said Trump will be "held accountable" for his administration's refusal to cooperate.
"The White House should be warned that the continued efforts to hide the truth of the President's abuse of power from the American people will be regarded as further evidence of obstruction," Pelosi said.
The correspondence from Trump's office came shortly after the State Department abruptly ordered Gordon Sondland, the U.S. ambassador to the European Union, not to appear before a congressional panel as part of its probe of whether Trump abused his office for personal political gain.
In response, House Intelligence Committee Chairman Adam Schiff (Democrat-California) called Sondland's absence "additional strong evidence" of obstructing the congressional inquiry.
Sondland had already flown in from Europe to testify when Trump on October 8 called the probe a "kangaroo court" in a social media post.
Democratic lawmakers later in the day issued a subpoena to Sondland, compelling him to testify on October 16.
The inquiry was prompted by a government whistle-blower complaint about a July 25 phone call in which Trump urged Ukrainian President Volodymyr Zelenskiy to investigate his political and potential electoral foe, former Vice President Joe Biden.
Sondland, a political appointee and long-time Republican donor, has no prior diplomatic experience.
Ulrich Brechbuhl, a State Department counselor, was also scheduled to give a deposition, but there was no immediate word on whether he would still do so.
Marie Yovanovitch, the former U.S. ambassador to Kyiv who was abruptly recalled to Washington in May, is scheduled to testify on October 11, although her appearance has been delayed once already.
The whistle-blower complaint claimed that Trump was "using the power of his office to solicit interference from a foreign country" in the 2020 U.S. election given that Biden is perceived as a leading Democratic candidate to face the incumbent Republican president.
Leading up to the call, Trump had withheld nearly $400 million in military aid and White House staff had later allegedly sequestered a record of the discussion between the two leaders in a computer server meant for highly classified material.
According to the whistle-blower complaint, Sondland and former special envoy to Ukraine Kurt Volker met twice with Ukrainian officials "to help Ukrainian leaders understand and respond to the differing messages they were receiving from official U.S. channels on one hand and from [Trump’s personal attorney] Mr. [Rudy] Giuliani on the other."
Trump has denied any wrongdoing and has called the impeachment inquiry a "scam" that "makes it harder to do my job."