The Justice Department’s inspector general, as part of a long-running internal review, is focusing on how the Democrat-funded anti-Trump dossier was used to secure surveillance warrants for a former Trump campaign adviser in 2016, despite questions about its credibility.

A source familiar with the matter confirmed to Fox News that Justice Department IG Michael Horowitz is probing how the dossier compiled by former British spy Christopher Steele was used to secure the original surveillance warrant for former Trump aide Carter Page in October 2016, as well as for three renewals.

The focus is part of Horowitz’s investigation into alleged surveillance abuses during the 2016 campaign.

The Wall Street Journal reported Wednesday that Horowitz is close to concluding his inquiry into the genesis of the Russia probe. The report said Horowitz’s team has questioned why the FBI considered Steele a credible source, and why the bureau seemed to use news reports to bolster Steele’s credibility.

With Special Counsel Robert Mueller’s Russia report now out in the open, attention has returned to the salacious and unverified anti-Trump dossier authored by Steele -- especially since its more sensational claims were not substantiated by Mueller's team.

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Former FBI Director James Comey has also faced renewed scrutiny for his role, as the inspector general as well as Attorney General Bill Barr examine the origins of the probe.

Barr testified recently that "spying" did occur against the Trump campaign.

Comey dismissed Barr’s comments at the time, saying he “never thought of” electronic surveillance as “spying.”

But Comey was put on the defensive again following a recent New York Times report detailing FBI efforts to investigate the Trump campaign.

The FBI reportedly sent a woman to meet with then-adviser George Papadopoulos at a bar in London during the campaign. The woman, who identified herself as Azra Turk, asked Papadopoulos point-blank if Trump was collaborating with Russians to swing the 2016 election.

Trump himself singled out Comey in an interview last week with Fox News.

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“Look, I think what they should be focusing on is how did this mess start?” Trump told Fox News' Catherine Herridge. “How did this whole investigation start, because I think it’s corrupt as hell.”

Of Comey, he said: "I think that he did a terrible job. I would say he probably ... led some kind of an effort. The word spying has been used. He probably was one of the people leading the effort on spying."

For his part, Comey has been promoting his anti-Trump book and is set to participate in a CNN town hall on Thursday night.

Fox News’ Lukas Mikelionis contributed to this report.