Sen. Chuck Grassley Charles (Chuck) Ernest GrassleySenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE (R-Iowa) is pressing the FBI to explain how it sought to ensure that foreign intelligence used in its Russia investigation was not ultimately sourced back to a controversial and unverified dossier alleging ties between the Trump campaign and Russia.

In an Oct. 4 letter to FBI Director Christopher Wray, Grassley, the chairman of the Senate Judiciary Committee, voiced concern over "material inconsistencies" in the FBI's answers to past questions about its use of the dossier authored by former British spy Christopher Steele.

Citing reports that Steele had passed at least some of the dossier's contents to the British government, Grassley asked how the FBI ensured that the foreign intelligence used in its Russia investigation was not based solely on Steele's research.

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"If this in fact happened, it would be alarming," Grassley wrote. "Mr. Steele’s dossier allegations might appear to be 'confirmed' by foreign intelligence, rather than just an echo of the same 'research' that Fusion bought from Steele and that the FBI reportedly also attempted to buy from Steele."

Steele, a former MI6 officer, was hired last year by the private research firm Fusion GPS to compile the dossier, which details unverified allegations of possible collusion between the Trump campaign and Russia.

Grassley's letter asks the FBI to provide the Judiciary Committee with any foreign intelligence reports that have been used in the agency's Russia investigation, as well as to explain what steps it took to determine whether Steele was the source behind that foreign intelligence.

The dossier had circulated throughout Washington during the 2016 presidential election. But most news organizations declined to report on it, because its contents — including many salacious allegations — were unable to be verified.

Former FBI Director James Comey briefed President Trump on the dossier weeks before he took office. The president has repeatedly denied the information put forth in the document.

Special counsel Robert Mueller, who is conducting the criminal investigation into Russia's efforts to interfere in the 2016 election, has reportedly met with Steele as part of the probe.