A criminal investigation has been opened into an FBI official suspected of 'altering' a document in the warrant application to surveil Trump campaign advisor Carter Page during the 2016 election, according to a new report.

The alleged document tampering was uncovered during Inspector General Michael Horowitz's probe into the origins of the Trump-Russia investigation, and subsequently referred to U.S. Attorney John Durham for criminal investigation, CNN reported on Thursday.

In October 2016, the FBI sought and was granted a FISA warrant to surveil Page's communications and access all of his stored emails and texts.

To obtain the warrant, the FBI had to convince a judge in a secret court that there was probable cause that Page was a foreign agent. The 90-day warrant was renewed three times, but Page has never been charged with any crime.

Former Trump campaign advisor Carter Page is seen above. A new report says an FBI official is being investigated for tampering with documents used to obtain the FISA warrant against him

U.S. Attorney John Durham (above) has opened a criminal investigation into the origins of the Trump-Russia probe. An FBI official is now reportedly a subject in Durham's investigation

It's unclear which FBI official is under criminal investigation for allegedly tampering with documents in the warrant application.

The names of the supervisory special agent or agents who submitted sworn statements to support the FISA application and renewals were redacted when the applications were publicly released.

Former FBI director James Comey signed off on the FBI evidence in the applications prior to being fired by President Donald Trump in May 2017.

Former FBI Deputy Director Andy McCabe signed off on the subsequent renewals.

The alleged FBI document tampering is expected to be addressed in Horowitz's IG report, which is undergoing final classification review.

In a letter sent on Thursday to Senate Judiciary Committee Chairman Lindsey Graham, Horowitz said he expected his office to be able to release the report on December 9 'barring unforeseen circumstances.'

Former FBI director James Comey (left) and former FBI Deputy Director Andy McCabe (right) both signed off on FBI evidence in the FISA warrant application and renewals targeting Page

Inspector General Michael Horowitz (above) says he expects to release his report into the origins of the Trump-Russia investigation on December 9

Attorney General Bill Barr has pledged to make the report public with only the redactions needed to protect classified national security interests.

The report is expected to examine how closely the FBI stuck to the law and rules when it went to a secret Foreign Intelligence Surveillance Court beginning in 2016 to obtain authorization to conduct electronic monitoring of Page.

The report will also examine how British ex-spy Christopher Steele's 'dirty dossier' became key FBI evidence.

The dossier, which was funded by Trump's Democratic political opponents, was cited prominently in the FISA warrant application to surveil Page.

Steele's dossier claimed that Page was a secret intermediary to the Russian government and had personally hatched the idea to leak stolen DNC emails during the Democrats' nominating convention. Those claims have never been proven, and Page strenuously denies them.

Steele was questioned at length last summer by representatives of the Inspector General's office in connection with the forthcoming report.

The IG report will also examine how British ex-spy Christopher Steele's (above) 'dirty dossier' became key FBI evidence, including in the Page FISA application

Page, a one-time foreign policy adviser to Trump's 2016 campaign, recently sued the Justice Department, accusing it of violating his privacy by failing to give him an opportunity to examine the IG report before publication.

Durham, the chief federal prosecutor in Connecticut, is conducting a review of whether U.S. spy and law enforcement agencies acted properly when they initiated and pursued investigations of possible ties between Trump and his campaign and Russia.

A person familiar with the matter confirmed in late October that Durham's inquiry had become a criminal investigation.

Durham has visited Italy as part of his probe, reportedly to try to dig up information on Josef Mifsud, a mysterious Maltese academic.

John Durham has reportedly been trying to learn more about Josef Mifsud (above), a mysterious Maltese academic who indirectly triggered the Trump-Russia probe

It was Mifsud's tip to Trump presidential campaign advisor George Papadopoulos that Russia had 'dirt' on Hillary Clinton which triggered the FBI investigation into possible Trump campaign ties to Russia.

FBI insiders call Mifsud a Kremlin agent — but Trump supporters allege that he has ties to Western intelligence, including MI6, and claim he may have been a plant to incriminate the Trump campaign.

Mifsud hasn't been seen publicly for months, but last week an Italian publication received an encrypted file with a voice recording claiming to be Mifsud speaking on November 11.

In the recording, the voice denied being an agent of any national intelligence service, insisting he was merely an academic.