House Intelligence Committee ranking member Devin Nunes, R-Calif., demanded answers Saturday from the Intelligence Community Inspector General’s office regarding the whistleblower complaint about President Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky.

Nunes sent a letter to ICIG Michael Atkinson raising several questions about the complaint, which ultimately led to Trump's impeachment, and repeated requests for information that he said went unanswered for months. While several officials met for closed-door sessions to answer questions following the complaint, Atkinson’s testimony has not been released to the public.

“He’s the only one of all the star chamber games that were played in the basement of the Capitol, with the secretive interviews. The only one that’s not released is the one with the IC Inspector General. That’s unacceptable,” Nunes told Fox News’ “Sunday Morning Futures.”

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Nunes, along with Rep. Jim Jordan, R-Ohio, and House Minority Leader Kevin McCarthy, R-Calif., previously had sent a letter to Atkinson in September 2019 in which they raised a number of issues related to the whistleblower’s complaint. Nunes’ new letter claimed Atkinson’s office has not responded satisfactorily.

Among Nunes’ main concerns: the decision to revise a form for whistleblower complaints that removed the requirement of first-hand information in order for a complaint to be relayed to Congress.

Nunes’ September letter had inquired about the update to the form that had left out the first-hand knowledge requirement, and how it had been dated August 2019 despite evidence that it was created on Sept. 24, 2019. Atkinson’s office later claimed that the form had been backdated in error because it had received preliminary approval in August. Now, Nunes is asking that if that was the case, why it took until late September for it to be posted alone.

“What he’s claiming is, essentially, ‘We’re just dumb, we made mistakes, it was a huge mistake,’” Nunes said Sunday. “That’s fine if you want to claim incompetence, but you need to have the documentation, the evidence to prove that you were indeed incompetent.”

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Nunes’ letter asked for information regarding all revisions to the form since May 24, 2018, as well as who approved them and copies of each revised version. Prior to then, the form stated that first-hand knowledge was required.

After media reports first noted the form change, Atkinson said in a lengthy statement that the whistleblower had actually filled out the older version of that form, which retained the requirement that whistleblowers have first-hand information. The ICIG revealed that the whistleblower had said he or she had first-hand information, as well as second-hand information, but it was unclear what the first-hand information was.

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The letter also sought ICIG policies regarding “the criteria for making a credibility determination” in cases where complaints have been deemed an “urgent concern,” such as the complaint about Trump’s call.

Nunes said he specifically wanted to know if Atkinson’s office changed its assessment of the whistleblower’s credibility in light of “incorrect or incomplete information” provided.

Fox Business Network's Maria Bartiromo contributed to this report.