This week marks one year since BuzzFeed’s publication of the controversial “Russia dossier,” the document compiled by the former British spy Christopher Steele that claimed the Russian government had been “cultivating, supporting, and assisting” Donald Trump for years, and that the Kremlin was holding incriminating material about Trump, including a sex tape from a Moscow hotel room.

In the past twelve months, we’ve learned a good deal more about the Steele dossier, including that it was commissioned by the Washington-based intelligence firm Fusion GPS, which received funding from the Democratic National Committee and Hillary Clinton’s Presidential campaign. Some of the allegations in the dossier, including the one about the sex tape, haven’t been corroborated, and Trump has described it as a “Crooked Hillary pile of garbage.” (In publishing the dossier, BuzzFeed noted that the allegations it contained were unverified and that it contained errors.)

However, we have also learned that the F.B.I. took Steele’s work seriously enough to interview him before the 2016 election. After the election, according to the Washington Post, the Bureau agreed to pay Steele to do some more research, but that agreement was cancelled after BuzzFeed published the dossier. There have also been unconfirmed reports that the Bureau used the dossier to obtain secret warrants to tap the phone of Paul Manafort, Trump’s former campaign manager. And, last January, James Comey, while he was still the F.B.I. director, informed President-elect Trump about the contents of the dossier.

In recent weeks, Republicans on the Senate Judiciary Committee, who are supposed to be investigating Russian interference in the 2016 election, have raised more questions about the dossier and the role it played in the F.B.I.’s Russia investigation. Last week, in a letter to the Justice Department, Charles Grassley, the head of the committee, and Lindsey Graham, another Republican member, asked the department to investigate Steele for possibly lying to the F.B.I., but they didn’t provide any evidence to back up the request. This looked like another G.O.P. effort to undermine the credibility of the Russia investigation, and, on Tuesday, Dianne Feinstein, the senior Democrat on the Judiciary Committee, responded by releasing a transcript of a lengthy interview that the committee’s lawyers carried out, last August, with Glenn Simpson, the co-founder of Fusion GPS. Although Simpson refused to answer some of the questions put to him, particularly those relating to his clients, his testimony provides new details about the genesis of the Steele dossier and Steele’s contacts with the F.B.I. It also provides a glimpse into an aspect of modern political campaigns that usually remains hidden: paid opposition research.

Simpson, who founded Fusion GPS in 2010, began his testimony by explaining his background as an investigative reporter at the Wall Street Journal and at Roll Call. He described himself as a “document hound” who specializes in searching public records. Fusion GPS doesn’t do much political work, he said, because “most campaigns don’t have the budget for the kind of services we provide.” Normally, his outfit worked for law firms on cases related to “financial crime, money laundering and fraud investigations, tax evasion, that sort of thing.”

But in September or October of 2015—Simpson was vague about the exact date—a client hired Fusion GPS to investigate Donald Trump. (The Washington Free Beacon, a conservative Web site that has received funding from the hedge-fund billionaire Paul Singer, has acknowledged hiring Fusion GPS to investigate candidates during the Republican Presidential primaries.) Simpson said the mandate was to carry out an “open-ended” examination of Trump’s business record, including his bankruptcies, and, he added, “it evolved somewhat quickly into issues of his relationship to organized-crime figures.”

One of the early subjects of the inquiry, Simpson said, was Felix Sater, a longtime business associate of Trump’s who reportedly was linked to the Russian mafia, and who played a role in the development of the Trump SoHo building. Another subject of interest was the financing of Trump’s various real-estate projects. “We saw indications that some of the money came from Kazakhstan, among other places, and that some of it you just couldn’t account for,” Simpson said. But, for the first six months of the work, he added, the probe wasn’t focussed only on Russia.

Things changed in the spring of 2016, after Fusion GPS got a new client, which we now know to have been the Clinton campaign and the D.N.C. (This fact emerged in October, well after Simpson’s testimony, when lawyers from the Judiciary Committee demanded Fusion GPS’s bank records.) In “May or June of 2016,” Simpson recalled, he engaged Christopher Steele, an old associate of his, who was the former head of the Russia desk at the British foreign-intelligence agency, MI6. He and Steele, who was by then running his own intelligence consultancy in the U.K., shared an interest in the Russian kleptocracy and in organized-crime issues, Simpson said.

Asked about the methods Steele used to compile his reports, Simpson said that, rather than visiting Moscow himself, Steele relied on “a network of people, sources” that he had in Russia, which gathered information for him. “What people call the dossier is not really a dossier,” Simpson said. “It’s a collection of field memoranda, of field interviews, a collection that accumulates over a period of months . . . . He’d reach a point in the reporting where he had enough to send a new memo; so he’d send one.” In response to a question about whether Steele paid any of his sources, Simpson said that he hadn’t asked him that question.

In any case, when Steele sent in his first memorandum, which was thirty-five pages long and dated June 20, 2016, it contained some explosive allegations, including claims that the Russian regime had been carefully cultivating Trump, and that the F.S.B., the Kremlin’s domestic-intelligence agency, had “compromised TRUMP through his activities in Moscow sufficiently to be able to blackmail him.” The memorandum also quoted Steele’s “Source A . . . a senior Russian Foreign Ministry figure,” as saying, “the Kremlin had been feeding TRUMP and his team valuable information on his opponents, including Democratic presidential candidate Hillary CLINTON, for years.”

It was about this time, Simpson said, that Steele first contacted the F.B.I. In Simpson’s telling, taking this step was Steele’s idea. Shortly after filing his first memo, Simpson recounted, “Chris said he was very concerned about whether this represented a national-security threat and said he wanted to—he said he thought we were obligated to tell someone in government, in our government, about this information. He thought from his perspective there was an issue—a security issue about whether a Presidential candidate was being blackmailed.” Simpson said that he didn’t agree or disagree with Steele’s suggestion, but said he’d think about it. “Then he raised it again with me. I don’t remember the exact sequence of these events, but my recollection is that I questioned how we would do that because I don’t know anyone there that I could report something like this to and be believed, and I didn’t really think it was necessarily appropriate for me to do that. In any event, he said, ‘Don’t worry about that, I know the perfect person, I have a contact there, they’ll listen to me, they know who I am, I’ll take care of it.’ I said O.K.”

It was in early July, 2016, that Steele spoke with his F.B.I. contact and relayed the Russia allegations, Simpson said. After that, Steele continued his work for Fusion GPS, which led to more memos, including one that addressed the activities of Carter Page, a foreign-policy adviser to the Trump campaign. To Simpson’s chagrin, however, neither Steele nor Fusion GPS heard anything more from the F.B.I. for months. During that time, Simpson pointed out, the hacking of the D.N.C. was revealed, the Republican Party’s platform was changed to be friendlier to Russia on the issue of Ukraine, and Trump continued to speak positively about Vladimir Putin. “So I vaguely recall that these external events prompted us to say, I wonder what the F.B.I. did, whoops, haven’t heard from them. . . . That was basically the state of things through September,” Simpson said.