The FBI's decision to use the unverified "dossier" about President Trump in a top-secret spy court that authorized surveillance on the Trump campaign may have been a crime, according to a letter obtained by Fox News.

The dossier contains the salacious claims that were assembled by a former British spy using Russian sources for a company linked politically to and funded by the Hillary Clinton campaign in 2016.

The dossier was the basis for the FBI's investigation of Trump's campaign, including special counsel Robert Mueller's so-far failed hunt for evidence of collusion between the Trump campaign and Russia.

Fox reported House Intelligence Committee chief Rep. Devin Nunes, R-Calif., wrote to Attorney General Jeff Sessions that the FBI, "in this instance," violated "basic operating guidance."

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"The FBI may have violated criminal statutes, as well as its own strict internal procedures, by using unverified information during the 2016 election to obtain a surveillance warrant on onetime Trump campaign aide Carter Page, the chairman of the House Intelligence Committee charged Thursday," Fox reported.

The congressman, who earlier released a summary of secret information that was used by the establishment in Washington to monitor the Trump campaign, cited the Domestic Investigations and Operations Guide.

That was set up by the FBI and endorsed by the Justice Department.

A 2011 version of the operations guide states that the "accuracy of information contained within FISA applications is of utmost importance... Only documented and verified information may be used to support FBI applications to the court."

But the dossier, which the Democrats have conceded was used as support for a FISA court search authorization, was unverified.

"The guidance also states that information in a FISA application must be 'thoroughly vetted and confirmed,'" Fox reported. "A memo released by the intelligence committee last month stated that the FBI and DOJ sought surveillance warrants to spy on [Carter] Page by using an infamous, Democrat-funded anti-Trump dossier that included salacious and unverified allegations about Trump's connections to Russia."

Nunes listed five "criminal statutes" that may have been violated, including contempt, obstruction and conspiracy.

The letter also cites the crime of willfully depriving a person of a right protected by the Constitution.

"Former and current DOJ and FBI leadership have confirmed to the committee that unverified information from the Steele dossier comprised an essential part of the FISA applications related to Carter Page," Nunes wrote Thursday.

Nunes' letter seeks an answer from Sessions whether those protocols, that demand verified information only, have been changed. And if not, what is the DOJ doing to hold those who violated the requirements responsible.

Democrats attacked the messenger.

Adam Schiff, the ranking Democrat on Nunes' committee, said it was just another effort "to attack and malign the honorable men and women of the FBI."

A DOJ spokesman told FOX the inspector general would be looking into the issue.

It was ex-FBI chief Jim Comey, who later was fired by President Trump, who confirmed that the dossier, which is now known to have been used by the Obama administration in the FISA court, was "unverified" and "salacious."

The Washington Examiner explained Nunes has "worked to secure the release of a memo, put together by Rep. Trey Gowdy, R-S.C., and staff, which outlined alleged government surveillance abuses. That memo, which last month was OK'd for release without redactions by President Trump over the objections of the FBI and DOJ, claimed 'Trump dossier' author Christopher Steele played an important role in the initial FISA application and all three renewal FISA applications to spy on Page. The dossier is filled with salacious and unverified claims about Trump's ties to Russia."

The memo prompted Schiff to write his own memo, claiming that the FBI and DOJ didn't abuse the Foreign Intelligence Surveillance Act process.