A top Republican threatened to involve the Justice Department if the Intelligence Community inspector general refuses to deliver "documentary evidence" involving the whistleblower whose complaint spurred the impeachment investigation into President Trump.

Rep. Devin Nunes, the ranking member on the House Intelligence Committee, gave Inspector General Michael Atkinson until Friday to hand over information that could help illuminate questions surrounding the timeline and procedural protocol used to vet the whistleblower complaint that centered on Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky, according to a Friday letter obtained by the Washington Examiner.

The California Republican decried what he called the "unusual handling" of the complaint process and said he will refer the matter to the Justice Department if Atkinson's office refuses to comply after months of outreach.

The Intelligence Community inspector general declined to comment on this story.

House Minority Leader Kevin McCarthy and Rep. Jim Jordan first raised concerns along with Nunes about the process in a Sept. 30 letter to Atkinson. The Republican trio found Atkinson's response "incomplete, vague, and/or evasive," and Nunes sent another letter to the IGIC office on Jan. 11 but again determined the inspector general's answers to be "equally unsatisfactory."

The whistleblower, believed by some to be career CIA analyst Eric Ciaramella, filed a complaint to Atkinson's office on Aug. 12. Atkinson determined the complaint to be "urgent" and credible, and with permission from the acting director of national intelligence, notified Congress of its existence, thus sparking what would become the impeachment fight. Trump was acquitted on two articles of impeachment by the Senate last week.

Nunes, a vocal defender of the president, has expressed particular concern with a change to the whistleblower complaint submission form that allowed for hearsay information to be included.

"The investigation is particularly focused on the guidelines that appeared on a whistleblower complaint submission form that was changed — after the submission of the whistleblower complaint — to eliminate language excluding hearsay information."

Republicans raised concerns about the timing of changes to the whistleblower complaint form, but Atkinson defended the filing process, claiming the intelligence community has the authority to launch investigations based on second-hand information and that the change in form had no direct connection to the timing of the whistleblower complaint on Aug. 12.

The Justice Department Office of Legal Counsel issued a statement saying it believed the whistleblower complaint did not possess an "urgent concern."