WASHINGTON – The inspector general who received the whistleblower’s complaint at the heart of the impeachment investigation of President Donald Trump described privately for the House Intelligence Committee on Friday how he determined it appeared credible.

Michael Atkinson, the inspector general for the intelligence community, received the whistleblower complaint Aug. 12 that focused on Trump’s call urging Ukraine President Volodymyr Zelensky to discredit a political rival, former Vice President Joe Biden. Trump has since released a summary of the call and the complaint.

After a nearly eight-hour meeting, Intelligence Committee Chairman Adam Schiff, D-Calif., said he couldn't describe the testimony in detail, but he thanked Atkinson for his professionalism.

"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," Schiff said in a statement. “We look forward to following up on what we learned today and continuing our investigation into the facts.”

Rep. Mike Quigley, D-Ill., told USA TODAY that Atkinson provided no new documents on Friday, but walked through the step-by-step process he took to deem the whistleblower credible.

“I don’t believe there was anything new revealed today,” Quigley said. “After yesterday, we need a break,” he added, referring to the nine-hour interview with former Ukraine envoy Kurt Volker.

Volker:Trump said Ukraine was full of 'terrible people' who 'tried to take me down'

Several Republicans left the meeting accusing Democrats of working with the whistleblower before the complaint was released.

“The inspector general for the intelligence community could provide no information about the contact between the (committee) majority and the whistleblower prior to his involvement," said Rep. John Ratcliffe, R-Texas. "The only way we’ll be able to get that information is from the (committee) majority themselves.”

Ratcliffe appeared with GOP Reps. Chris Stewart of Utah and Brad Wenstrup of Ohio, but neither of them answered questions.

A representative for the whistleblower asked the committee for guidance about how to file a complaint, but a panel spokesman said staffers directed the whistleblower to hire a lawyer to file a complaint with the inspector general. Mark Zaid, one of the lawyers on the whistleblower's legal team, said no one from the committee helped craft the complaint.

“Nevertheless, Republicans continued the president’s strategy of deflection by making the absurd claim that because a whistleblower contacted the committee seeking guidance, the committee cannot conduct an investigation into the complaint,'" Schiff said. "If that were true, no whistleblower could contact Congress, and no committee could conduct an investigation."

As House Democrats worked to corroborate the complaint, Trump told reporters outside the White House on Thursday that both Ukraine and China should investigate Biden.

"Well, I would think if they were honest about it, they'd start a major investigation into the Bidens. It's a very simple answer,” Trump said of Zelensky. “And by the way, likewise, China should start an investigation into the Bidens because what happened in China is just about as bad as what happened with Ukraine.”

Biden, a leading candidate for the Democratic nomination for president in 2020, has forcefully denied wrongdoing with his son, Hunter Biden, who worked on the board of an energy company in Ukraine. Kate Bedingfield, the communications director for Biden’s campaign, called Trump’s statement “a grotesque choice of lies over truth and self over country.”

Ukraine texts:Read key text messages between diplomats on Trump, Ukraine president

Atkinson spent two weeks reviewing the whistleblower's complaint that alleged Trump “used the power of his office” to solicit foreign help for the 2020 election and then restricted access to records of the call. Atkinson found that the complaint appeared credible Aug. 26, after a preliminary review.

But the acting director of national intelligence, Joseph Maguire, prevented him from passing along the complaint to Congress within seven days, as is typically required for national-security whistleblower complaints. Maguire consulted with the White House and Justice Department in opting for the delay, he told the intelligence panel Sept. 26, the day after a summary of the call was released and the complaint was provided to the committee.

Maguire said he thought the complaint might be protected by executive privilege, despite being a conversation with a foreign leader. The Justice Department ruled that the complaint didn’t qualify as an “urgent concern” about "a serious or flagrant problem" requiring notification of Congress because the target – Trump – isn’t a member of the intelligence community.

Atkinson respectfully disagreed with that decision, saying in a letter Sept. 17 that the allegations related to “one of the most significant and important” of Maguire’s responsibilities to the American people. Atkinson warned lawmakers that withholding the information could lead to “a significant problem and deficiency” relating to Maguire’s intelligence programs.

Trump released a memo summarizing the calland Maguire provided the complaint to the House and Senate intelligence committees on Sept. 25. Lawmakers on those panels now want to hear from the whistleblower.

Schiff has characterized the call as sounding like a mafia shakedown, seeking the favor of an investigation of Biden in exchange for the release of military aid. He called Maguire’s decision to consult with the White House “bewildering” during the Sept. 26 hearing. And Schiff said Wednesday that lawmakers will have questions for Atkinson because they didn't have the complaint when they last met with him.

“We certainly intend to ask (Atkinson) about the efforts that were made to corroborate that complaint, which we now know the inspector general found both credible and urgent," Schiff said.

But the top Republican on the panel, Rep. Devin Nunes of California, said the whistleblower had arguable political motivation for the complaint and said Democrats were using it as ammunition in political warfare.

Trump has called the whistleblower "very inaccurate" and Schiff "a lowlife."

"He should resign from office in disgrace and frankly they should look at him for treason," Trump said of Schiff on Wednesday while meeting with the president of Finland.

Six House committees have been pursuing wide-ranging investigations of Trump since Democrats regained control of the chamber in January. House Speaker Nancy Pelosi, D-Calif., announced a formal impeachment inquiry Sept. 24 based on reports about Trump’s Ukraine call.

The inspector general's meeting comes amid a flurry of subpoenas for documents and depositions with State Department officials and associates of Trump's personal lawyer, Rudy Giuliani, who met with Ukraine officials to urge an investigation of Biden.

More about Congress investigating President Trump's Ukraine call:

What's going on with Trump and Ukraine? And how does it involve Biden and a whistleblower complaint?

How does Congress hear from anonymous witnesses? Trump whistleblower seeks protection from retaliation

'Unique and unprecedented': Intel chief Joseph Maguire cites executive privilege in delaying whistleblower report to Congress

Nancy Pelosi has put the Trump impeachment inquiry on a fast track. Here's the plan, timeline and key players