Mr. Trump’s allies have sought to conflate the much broader Russia investigation with the dossier and have increased their attacks since the special counsel, Robert S. Mueller III, concluded that while the campaign welcomed and expected to benefit from Russia’s election interference, the evidence did not prove any conspiracy.

Investigators working for Mr. Horowitz have asked witnesses about whether the F.B.I. properly opened the Russia investigation and how the bureau handled a pair of informants, including Mr. Steele, whose work was financed by Mrs. Clinton’s presidential campaign and the Democratic National Committee. Reuters first reported the investigators’ interview of Mr. Steele. They have conducted more than 100 interviews and have begun drafting their report, Mr. Horowitz wrote to lawmakers last month.

Mr. Horowitz is expected to answer whether Mr. Steele’s information played a role in opening the Russia investigation, code-named Crossfire Hurricane. Former law enforcement officials have insisted it did not, saying they opened the inquiry in July 2016. The Steele dossier did not reach the relevant agents until Sept. 19, 2016, nearly two months later, people familiar with the matter have said.

But the primary focus of the inspector general’s inquiry is the role that Mr. Steele’s information played in investigators’ effort to obtain court permission to wiretap Carter Page, a Trump campaign foreign policy adviser. The Foreign Intelligence Surveillance Court approved the application on Oct. 21, 2016, about a month after Mr. Page had left the campaign, and the wiretap order — one of 1,559 the court issued that year — was renewed three times in 2017, including twice by Trump-era Justice Department officials.

In August 2016, a month before agents on the Russia investigation received the Steele dossier, they had already started discussions with the Justice Department about seeking a wiretap order targeting Mr. Page, according to people familiar with the investigation’s timeline. Agents identified Mr. Page as a potential conduit between the campaign and Moscow, if there was any, because he had close business ties to Russia, had traveled there in July after joining the campaign, and had been targeted as a potential recruit by Russian intelligence agents in 2013 — yet did not seem concerned when the F.B.I. talked to him about it.

Still, the arrival of the dossier in September kicked the deliberations over whether to seek a wiretap order into a higher gear, according to people familiar with the Russia investigation. By adding further weight to their reasons to be suspicious of Mr. Page, Mr. Steele’s information helped officials overcome bureaucratic reluctance stemming from fears that any leak of the existence of such a wiretap would be politically radioactive.