Two Republican senators are asking the Department of Justice to open a criminal investigation into Christopher Steele, the author of a controversial opposition research dossier on President Trump.

Sens. Chuck Grassley (Iowa) and Lindsey Graham (S.C.) sent a letter to Deputy Attorney General Rod Rosenstein and FBI Director Christopher Wray asking that they investigate if the former British intelligence agent lied to federal authorities.

"I don't take lightly making a referral for criminal investigation. But, as I would with any credible evidence of a crime unearthed in the course of our investigations, I feel obliged to pass that information along to the Justice Department for appropriate review," Grassley said in a statement.

Graham added that "after reviewing how Mr. Steele conducted himself in distributing information contained in the dossier and how many stop signs the DOJ ignored in its use of the dossier, I believe that a special counsel needs to review this matter."

The letter is the first known criminal referral from any of the congressional investigations into Russia's election interference.

As part of their request, the two senators passed along a memo tied to "certain communications between Christopher Steele and multiple U.S. news outlets." They did not publicly release the memorandum, which is classified.

The referral is the latest sign of partisan splintering of the Judiciary Committee's wide-ranging investigation into Russia, the 2016 election and potential political interference of the FBI.

Sen. Dianne Feinstein (Calif.), the top Democrat on the panel, blasted the letter as the latest attempt by GOP lawmakers to turn the committee's focus away from Trump.

"I think this referral is unfortunate as it's clearly another effort to deflect attention from what should be the committee's top priority: determining whether there was collusion between the Trump campaign and Russia to influence the election and whether there was subsequent obstruction of justice," she said.

Feinstein said she and the other Democrats on the panel were not consulted about the referral.

The Justice Department isn't required to open up an investigation in response to Grassley and Graham's letter.

And Grassley's office, in announcing their request, noted that it's standard for the committee to notify the Justice Department when it "comes across what appears to be credible evidence of a criminal violation that warrants further investigation by appropriate authorities based on information from any source, public or non-public."

The referral comes days after the latest public back-and-forth between Grassley and Fusion GPS, the research firm that hired Steele to compile the controversial dossier.

The firm and Steele have been a major target of Grassley, who has sent a flurry of letters trying to find out the extent of the FBI's contact with Steele as well as who Steele's sources are.

A lawyer for Fusion GPS fired back at the referral, saying that "we should all be skeptical in the extreme."

"Publicizing a criminal referral based on classified information raises serious questions about whether this letter is nothing more than another attempt to discredit government sources, in the midst of an ongoing investigation," the firm's counsel said.

The dossier has become a flashpoint in the Russia investigations, with Republicans questioning whether the document was the starting point for the FBI's investigation of the Trump campaign.

Some of the allegations in the dossier have been shown to be false, while others have either been supported by public evidence or remain unproven.

The Justice Department referral, according to Grassley's office, is not tied to the "veracity of claims contained in the dossier" and is not "intended to be an allegation of a crime."

"Maybe there is some innocent explanation for the inconsistencies we have seen, but it seems unlikely. In any event, it's up to the Justice Department to figure that out," Grassley added in his statement.

Steele, a former British intelligence officer, rocketed into the political spotlight in early 2017 after the Wall Street Journal reported that he was the author of the Trump dossier.

The FBI reportedly agreed to pay Steele to continue investigating Trump's ties to Moscow after the U.S. intelligence community concluded that the Kremlin had run an influence campaign to sway the presidential election in Trump's favor.

But those payments never materialized after Steele's identity was compromised.

CNN reported last last year that special counsel Robert Mueller has spoken to Steele as part of his investigation into potential collusion between the Trump campaign and Russia.

This story was updated at 2:53 p.m.