WASHINGTON — The 35-page dossier, spiced up with tales of prostitutes and spies, sketched out a hair-raising story more than two years ago. Russian intelligence had used bribery and blackmail to try to turn Donald J. Trump into a source and ally, it said, and the Kremlin was running some Trump campaign aides practically as agents.

But the release on Thursday of the report by the special counsel, Robert S. Mueller III, underscored what had grown clearer for months — that while many Trump aides had welcomed contacts with the Russians, some of the most sensational claims in the dossier appeared to be false, and others were impossible to prove. Mr. Mueller’s report contained over a dozen passing references to the document’s claims but no overall assessment of why so much did not check out.

Now the dossier, financed by Hillary Clinton’s campaign and compiled by a former British intelligence agent, Christopher Steele, is likely to face new, possibly harsh scrutiny from multiple inquiries.

Republicans in Congress have vowed to investigate. The Justice Department’s inspector general is considering whether the Federal Bureau of Investigation improperly relied on the dossier in applying to the secret Foreign Intelligence Surveillance Court for a warrant to eavesdrop on Carter Page, a Trump adviser. The inspector general wants to know what the F.B.I. learned about Mr. Steele’s sources and whether it disclosed any doubts about their veracity to the court.

And Attorney General William Barr has said he will review the F.B.I.’s conduct in the Russia investigation. His remark that there was “spying” on the Trump campaign has already encouraged Republican accusations of misconduct.

Interviews with people familiar with Mr. Steele’s work on the dossier and the F.B.I.’s scramble to vet its claims suggest that misgivings about its reliability arose not long after the document became public — and a preoccupation of Trump opponents — in early 2017. Mr. Steele has made clear to associates that he always considered the dossier to be raw intelligence — not established facts, but a starting point for further investigation.

By January 2017, F.B.I. agents had tracked down and interviewed one of Mr. Steele’s main sources, a Russian speaker from a former Soviet republic who had spent time in the West, according to a Justice Department document and three people familiar with the events, who spoke on the condition of anonymity. After questioning him about where he’d gotten his information, they suspected he might have added his own interpretations to reports passed on by his sources, one of the people said. For the F.B.I., that made it harder to decide what to trust.

Agents did not believe that either the source or Mr. Steele was deliberately inventing things, according to the former official. How the dossier ended up loaded with dubious or exaggerated details remains uncertain, but the document may be the result of a high-stakes game of telephone, in which rumors and hearsay were passed from source to source.

Another possibility — one that Mr. Steele has not ruled out — could be Russian disinformation. That would mean that in addition to carrying out an effective attack on the Clinton campaign, Russian spymasters hedged their bets and placed a few land mines under Mr. Trump’s presidency as well.

Oleg D. Kalugin, a former K.G.B. general who now lives outside Washington, saw that as plausible. “Russia has huge experience in spreading false information,” he said.

Mr. Steele declined to comment for this article. But Joshua A. Levy, a lawyer for Fusion GPS, the firm that commissioned the dossier, said the Mueller inquiry substantiated “the core reporting” in the Steele memos — including “that Trump campaign figures were secretly meeting Kremlin figures,” and that Russia’s president, Vladimir V. Putin, had directed “a covert operation to elect Donald J. Trump.”

While The New York Times and many other news organizations published little about the document’s unverified claims, social media partisans and television commentators discussed them almost daily over the past two years. The dossier tantalized Mr. Trump’s opponents with a worst-case account of the president’s conduct. And for those trying to make sense of the Trump-Russia saga, the dossier infused the quest for understanding with urgency.

In blunt prose, it suggested that a foreign power had fully compromised the man who would become the next president of the United States.

The Russians, it asserted, had tried winning over Mr. Trump with real estate deals in Moscow — which he had not taken up — and set him up with prostitutes in a Moscow hotel in 2013, filming the proceedings for future exploitation. A handful of aides were described as conspiring with the Russians at every turn.

Mr. Trump, it said, had moles inside the Democratic National Committee. The memos claimed that he and the Kremlin had been exchanging intelligence for eight years and were using Romanian hackers against the Democrats, and that Russian pensioners in the United States were running a covert communications network.

Such shocking claims may have seemed more plausible because of the conduct of Mr. Trump and his advisers. He was outspoken in his praise for Mr. Putin and hostile toward NATO. And a dozen associates, including his son Donald Jr., met or corresponded with Russians, including some with suspected intelligence connections, while failing to report the contacts to the F.B.I.

Mr. Trump’s former lawyer Michael D. Cohen negotiated for a Trump Tower project in Moscow many months into the campaign — and later admitted lying about it to Congress, along with tax evasion and other crimes. But Mr. Cohen did not, as the dossier claimed, travel to Prague to conspire in the Russian hacking of Democrats, the Mueller report makes clear.

Similarly, Mr. Page, a foreign policy adviser, was invited to address a prestigious Moscow institute in July 2016 in what seems to have been a calculated Russian attempt to curry favor. But Mr. Mueller, after a two-year investigation involving 40 F.B.I. agents, provided no evidence to support the claim that the adviser had collected a brokerage fee for the sale of a share of the Russian oil giant Rosneft. Nor has any evidence emerged to support the dossier’s claims about D.N.C. moles, Romanian hackers, Russian pensioners — or years of Trump-Putin intelligence trading.

Other dossier assertions remain neither proved nor disproved, notably its claim about Mr. Trump’s alleged dalliance with prostitutes. The Mueller report says a Russian businessman texted Mr. Cohen a week before the election to say that he had “stopped the flow of tapes from Russia but not sure if there’s anything else.” The businessman, Giorgi Rtskhiladze, later said he was referring to compromising tapes of Mr. Trump — but had been told they were fake.

The dossier began as part of a conventional opposition research operation by a small Washington firm, Fusion GPS. During the early part of the campaign, Fusion was paid by The Washington Free Beacon, a conservative publication funded by the billionaire Paul Singer, to scrutinize Mr. Trump, with the evident goal of uncovering dirt to help his Republican primary opponents.

After Mr. Trump emerged as the likely nominee, Fusion kept working but turned to a new source of funding: the law firm representing the Clinton campaign, Perkins Coie. Noticing in May 2016 the Trump campaign’s unexpected affinity for Russia, Fusion hired Mr. Steele, a veteran of Britain’s MI6 intelligence agency, to dig deeper. Mr. Steele has told acquaintances that he did not know the ultimate client was the Clinton campaign.

Former F.B.I. and Central Intelligence Agency officers who knew him respected Mr. Steele, who had served in Moscow before leading the Russia desk at MI6’s headquarters. After leaving government service, he had helped the F.B.I. with an investigation of FIFA, the international governing body of soccer, and with other matters involving Russia.

Mr. Steele, who co-owns a private intelligence firm in London, Orbis Business Intelligence, reached out to sources he had relied on in past Russia-related investigations. Between June and December 2016, he sent Fusion at least 17 reports, ranging from one to three pages, describing raw, unconfirmed information on a wide range of alleged connections between Mr. Trump, his aides and Russian operatives.

Mr. Steele decided early on that what he was hearing posed a grave danger to the United States. He alerted an F.B.I. agent he knew well, and on Sept. 19, 2016, his reports reached the team in Washington that had started investigating Trump-Russia connections nearly two months earlier.

The F.B.I. assembled a group of analysts to check every line of Mr. Steele’s short memos. Agents hit the streets to find and interview his sources, eventually identifying and speaking with at least two.

By summer 2017, with Mr. Mueller’s investigation in high gear, the F.B.I. still could not vouch for much of the dossier. One often-discussed claim — the detailed account of Mr. Cohen’s supposed trip to Prague — appeared to be false. Mr. Cohen’s financial records and C.I.A. queries to foreign intelligence services revealed nothing to support it.

F.B.I. agents on Mr. Mueller’s team debriefed Mr. Steele himself in London for two days in September 2017, according to a person familiar with the meeting.

Last year, in a deposition in a lawsuit filed against Buzzfeed, Mr. Steele emphasized that his reports consisted of unverified intelligence. Asked whether he took into account that some claims might be Russian fabrications, he replied, “Yes.”

F.B.I. agents considered whether Russia had polluted the stream of intelligence, but did not give it much credence, according to the former official.

But that is an issue to which multiple inquiries are likely to return. There has been much chatter among intelligence experts that Mr. Steele’s Russian informants could have been pressured to feed him disinformation.

Daniel Hoffman, a former C.I.A. officer who served in Moscow, said he had long suspected the dossier was contaminated by Russian fabrications. The goal, he said, would be to deepen American divisions and blur the line between truth and falsehood.

“How many times have hearings on Capitol Hill used information from the dossier?“ Mr. Hoffman said. “How much damage has it already caused?”

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