WASHINGTON — Michael Atkinson, the intelligence community inspector general, was grilled behind closed doors on Capitol Hill Friday over his assessment that the whistleblower complaint now at the center of House Democrats' impeachment inquiry was credible.

"While we cannot get into the substance, we explored with the IG through documents and testimony the reasons why he found the whistleblower complaint to be both urgent and credible," House Intelligence Chairman Adam Schiff, D-Calif., said in a statement after committee members heard roughly seven hours of testimony.

Schiff said that based on the summary of the call the White House released last week detailing the phone conversation between President Donald Trump Trump and Ukrainian President Volodymyr Zelenskiy, it is apparent Atkinson's determination was "correct."

"That call record shows that Trump pressured a foreign leader to interfere in the 2020 election by investigating a political opponent. Those facts cannot be seriously contested," Schiff said.

Republicans, meanwhile, are embracing Trump's strategy of "deflection," Schiff said, "by making the absurd claim that because a whistleblower contacted the committee seeking guidance, the committee cannot conduct an investigation into the complaint."

"If that were true, no whistleblower could contact Congress, and no committee could conduct an investigation," he said.

Trump and another Republicans have accused Schiff — without evidence — of orchestrating the complaint after it was reported that his aide was contacted by the whistleblower for consultation before the whistleblower filed the complaint within the intelligence community.

The future whistleblower, a CIA officer, came to the committee after he had already filed a complaint with the CIA general counsel, and was concerned that the complaint was not being properly handled, Democratic committee aides said.

Democratic committee aides told NBC News that what happened was rather routine, and no different from the two to three times a month an intelligence agency employee comes to them with concerns.

They said they did what they usually do in that situation: They instructed the future whistleblower to file a formal document with the inspector general, as called for in the law.

Rep. John Ratcliffe, R-Texas, told reporters Friday that he thought Schiff should be "disqualifiedfrom running an investigation where his committee — members or staff — are fact witnesses about contact with the whistleblower and the whistleblower process." He added that Atkinson "can provide no information" about the contact committee Democrats had with the whistleblower prior to Atkinson's involvement.

Rep. Devin Nunes, R-Calif., ranking member on the Intelligence panel, declined several times to speak to reporters about Atkinson's testimony. Atkinson also declined to answer questions from reporters.

This marked the first time Atkinson had testified before the committee since last week’s public release of the whistleblower complaint, which focuses on Trump's efforts to pressure Ukraine to investigate former Vice President Joe Biden and his son, Hunter. Atkinson briefed the committee behind closed doors earlier last month about the complaint, but didn't reveal details about its substance at the time, NBC News reported.

Upon emerging from Friday's deposition, Illinois Democratic Reps. Mike Quigley and Raja Krishnamoorthi said they found Atkinson's testimony useful.

"Today is very helpful in terms of fleshing out what we know about his investigation," Krishnamoorthi told reporters. "He ran his own investigation to corroborate the whistleblower’s complaint and talk to witnesses and so forth. So we learned substantively what he learned."

The congressman said Atkinson's testimony "reinforces" what lawmakers had learned about a July phone call between call between Trump and Zelenskiy. In the call, Trump asked the Ukrainian president to help investigate the Biden family's business dealings as well as matters related to the 2016 election, according to a detailed record released by the White House. There is no evidence of wrongdoing by either of the Bidens.

"What the inspector general said last time was the whistleblower pulled the fire alarm," Quigley said. "We have now seen the smoke and the fire."

Quigley added that Atkinson "has a priority, as ours is, to protect the whistleblower."

The whistleblower's complaint alleges that White House officials tried to “lock down” all records of the July 25 phone call between Trump and Zelenskiy because they were so concerned about what Trump said during the conversation.

The whistleblower, whose name and gender has not been released, lodged the complaint out of a stated belief that Trump was "using the power of his office to solicit interference from a foreign country" in the 2020 election.

Atkinson had deemed the complaint an "urgent concern" that he was required by law to provide to the congressional intelligence committees. But acting Director of National Intelligence Joseph Maguire refused to do so on the advice of the Justice Department, resulting in a standoff with Congress that ultimately resulted in House Speaker Nancy Pelosi backing the formal impeachment inquiry against Trump.

Atkinson, a former Justice Department lawyer appointed to this current position by Trump in 2017 and confirmed by the Senate last year, is appearing before the Intelligence Committee a day after the former U.S. special envoy to Ukraine, Kurt Volker, testified before several congressional committees about the administration's dealings with Ukraine.

Volker, who spoke to lawmakers for more than nine hours on Thursday, provided the committees with text messages that show U.S. ambassadors working to persuade Ukraine to publicly commit to investigating Trump’s political opponents and explicitly linking the inquiry to whether Ukraine’s president would be granted an official White House visit.

Atkinson’s deposition also comes as House Democrats prepare to issue a subpoena to the White House on Friday for failing to turn over records related to Ukraine that have been requested since early September.