FBI application for FISA Surveillance warrant did in fact largely rely on allegations from Steele anti-Trump dossier. When asked why FBI relied on anti-Trump dossier absent meaningful corroboration, then-director Comey cited Steele's credibility from past FBI dealings-US Senate Judiciary Committee letter, 1/4/18, 2/6/18 version with fewer redactions...(British subject and business owner Steele in 2016 was also a US political operative and FBI informant)

Christopher Steele is a British subject and British business owner. In 2016 he was also a US operative for Mrs. Clinton's campaign and an FBI informant providing alleged anti-Trump material.
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Jan. 4, 2018 letter, Feb. 6, 2018 version with fewer redactions, from US Senate Judiciary Committee, Sen. Grassley, Chairman, to US Deputy AG Rosenstein and FBI Director Wray, re: Referral of Christopher Steele for potential violation of 18 U.S. C. 1001   
 
page 2, page 4 pdf:

March 15, 2017: Then-FBI Director Comey briefs US Senate Judiciary Committee Chairman and Ranking Member:

(2nd parag.): "On March 15, 2017, the Chairman and the Ranking Member were provided copies of the two relevant FISA applications which requested authority to conduct surveillance of Carter Page. Both relied heavily on Mr. Steele's dossier claims, and both applications were granted by the Foreign Intelligence Surveillance Court (FISC).... 

When asked at the March 2017 briefing why the FBI relied on the dossier in the FISA applications absent meaningful corroboration--and in light of the highly political motives surrounding its creation--then-Director Comey stated that the FBI included the dossier allegations about Carter Page in the FISA applications because Mr. Steele himself was considered reliable due to his past work with the Bureau....

The bulk of the [FBI's Oct. 21, 2016] application consists of allegations against Page that were disclosed to the FBI by Mr. Steele and are also outlined in the Steele dossier. The application appears to contain no additional information corroborating the dossier allegations against Mr. Page, although it does cite to a news article that appears to be sourced to Mr. Steele's dossier as well....

page 3, page 5 pdf

The FBI stated to the FISC that "based on [Steele's] previous reporting history with the FBI, whereby [Steele] provided reliable information to the FBI, the FBI believes [Steele's] reporting to be credible." In short, it appears the FBI relied on admittedly uncorroborated information, funded by and obtained for Secretary Clinton's presidential campaign, in order to conduct surveillance of an associate of the opposing presidential candidate. It did so based on Steele's personal credibility and presumably having faith in his process of obtaining the information.

(U) But there is substantial evidence suggesting that Mr. Steele materially misled the FBI about a key aspect of his dossier efforts, one which bears on his credibility....

The FISA applications note the existence of a news article dated September 23, 2016, which in particular contained some of the same dossier information about Mr. Page compiled by Mr. Steele and on which the FBI relied in its application. While not explicitly stated, this is presumably the article by Michael Isikoff of Yahoo News, titled "U.S. Intel Officials Probe Ties between Trump Advisor and Kremlin."...The application attempts to explain away the inconsistency between Mr. Steele's  assertion to the FBI and the existence of the article, apparently to shield Mr. Steele's credibility, on which it still relied for the renewal request. The [FBI] application to the FISC said: "Given that the information contained in the September 23rd news article generally matches the information about Page that [Steele] discovered doing his/her research [page 4] the FBI does not believe that [Steele] directly provided this information to the press." (emphasis added)

Footnote "7. The FBI has failed to provide the [Judiciary] Committee the 1023s documenting all of Mr. Steele's statements to the FBI, so the Committee is relying on the accuracy of the FBI's representation to the FISC regarding those statements."

page 7, page 9 pdf, last paragraph on page:

"Mr. Steele's information formed a significant portion of the FBI's [FISA Court] warrant application, and the FISA application relied more heavily on Steele's credibility than on any independent verification or corroboration for his claims. Thus the basis for the warrant authorizing surveillance on a US citizen rests largely on Mr. Steele's credibility. The Department of Justice has a responsibility to [page 8] determine whether Mr. Steele provided false information to the FBI and whether the FBI's representations to the [FISA] court were in error."...







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