A lawyer for national security adviser John Bolton said that his client was involved in "many relevant meetings and conversations that have not yet been discussed" in the House's impeachment inquiry into President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's dealings with Ukraine.

Lawyer Charles Cooper wrote Friday in a letter obtained by The Hill that Bolton "was personally involved in many of the events, meetings, and conversations about which you have already received testimony, as well as many relevant meetings and conversations that have not yet been discussed in the testimonies thus far."

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Cooper also wrote in the letter — which was addressed to House general counsel Douglas Letter — that he was "dismayed" that the House committees did not attempt to compel Bolton and deputy national security adviser Charles Kupperman to testify after the men refused to testify voluntarily.

Bolton had been scheduled to testify Thursday, but did not appear. He is said to have expressed concern about attempts by Trump and his lawyer Rudy Giuliani to convince Ukrainian President Volodymyr Zelensky to look into Democratic presidential candidate Joe Biden, a former vice president, and has been portrayed as a potential key witness in the impeachment inquiry.

Cooper wrote that he had hoped to determine in court which takes precedence: House subpoenas to testify or the White House's assertion of executive privilege to prevent such testimony.

"We are dismayed that the Committees have chosen not to join us in seeking resolution from the Judicial Branch of this momentous Constitutional question as expeditiously as possible," Cooper wrote. "It is important both to Dr. Kupperman and to Ambassador Bolton to get a definitive judgment from the Judicial Branch determining their Constitutional duty in the face of conflicting demands of the Legislative and Executive Branches."

The House did not subpoena Bolton and withdrew a subpoena for Kupperman this week.

"Given the schedule of our impeachment hearings, a court process that leads to the dismissal of Dr. Kupperman’s flawed lawsuit would only result in delay, so we have withdrawn his subpoena," a House Intelligence Committee official said.

Cooper addressed this Friday, writing, "The House Chairs are mistaken to say Dr. Kupperman’s lawsuit is intended 'to delay or otherwise obstruct the Committees’ vital investigatory work.'"

Olivia Beavers contributed.