EXCLUSIVE: Republican Rep. Devin Nunes on Friday threatened to escalate his concerns about Intelligence Community Inspector General Michael Atkinson’s handling of the Ukraine whistleblower complaint to the Justice Department unless Atkinson complies with congressional requests for information and documents.

In a letter obtained by Fox News Friday, Nunes, the top Republican on the House Intelligence Committee, issued the stark warning to Atkinson, giving him a Feb. 14 deadline to comply with past congressional requests.

HOUSE INTEL REPUBLICANS INVESTIGATING ICIG HANDLING OF WHISTLEBLOWER COMPLAINT

“I will be referring this matter for investigation by the Department of Justice if you once again refuse to comply,” Nunes, R-Calif., wrote in the letter.

Nunes, House Republican Leader Kevin McCarthy, R-Calif., and Rep. Jim Jordan, R-Ohio, first penned a letter to Atkinson in September, requesting specific documents for investigation. Last month, Nunes sent another letter requesting documents that he claims are “well within this committee’s oversight responsibilities related to the Intelligence Community

House Intelligence Committee Republicans are investigating Atkinson’s “unusual handling" of the whistleblower complaint that formed the basis of President Trump's impeachment.

“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,” Nunes wrote Friday.

House Republicans, in their letters, had requested answers to why the revision on the complaint was changed and who was involved in the revisions.

Last month, Fox News reported that the committee was also investigating the veracity of Atkinson’s testimony about the whistleblower complaint and the explanations he offered for changes made to the intelligence community inspector general's guidance on accepting secondhand information.

GOP QUESTIONS MOVE TO DROP 'FIRST-HAND' INFO REQUIREMENT IN WHISTLEBLOWER FORM

In August, Atkinson received a complaint from an individual raising concerns about Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky in which he pressed Kiev to launch investigations into the dealings of former Vice President Joe Biden and his son Hunter Biden in Ukraine, as well as issues related to the 2016 presidential election.

The president’s request came after millions of dollars in U.S. military aid to Ukraine had been frozen, which Democrats argue shows a “quid pro quo” arrangement. Trump has denied wrongdoing.

Republicans for months have complained that the whistleblower made contact with Intelligence Committee Chairman Adam Schiff's, D-Calif., staff in advance -- though Schiff has downplayed the nature of that contact.

Whether Republicans are looking further into that contact as part of their review is unclear. But since last fall, they have specifically challenged intelligence community officials over changes to a key form that dropped a requirement for “firsthand information” in whistleblower complaints.

The White House released a declassified version of the complaint, which revealed that the whistleblower’s concerns stemmed from the secondhand accounts of “more than half a dozen U.S. officials.”

After the form change was first noticed in media reports, 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 firsthand information. The intelligence community inspector general revealed that the whistleblower had said he or she had firsthand information, as well as secondhand information, but it was unclear what the firsthand information was.

The declassified whistleblower complaint, though, stated: “I was not a direct witness to most of the events described. However, I found my colleagues’ accounts of these events to be credible, because, in almost all cases, multiple officials recounted fact patterns that were consistent with one another.”

Republicans have previously questioned whether the timing of the form revision was related to the Ukraine complaint.

Meanwhile, Atkinson found that the whistleblower showed indications of “political bias” and was “in favor of a rival political candidate,” while still deeming the complaint a matter of “urgent concern.”

The Justice Department Office of Legal Counsel later released a contrary opinion determining that the complaint did “not involve an ‘urgent concern’” and did not require that the director of national intelligence “transmit the complaint to the intelligence committees.”

On Wednesday, the Senate voted to acquit Trump on both charges against him stemming from the whistleblower complaint--abuse of power and obstruction of Congress.