The FBI opened a counterintelligence investigation into the Trump-Russia affair shortly after receiving the first installment of an anti-Trump dossier from a former British spy working for the Hillary Clinton campaign. What congressional investigators want to know is whether that was a coincidence or not.

The first report in the dossier compiled by former MI6 agent Christopher Steele was dated June 20, 2016.

Steele told the left-leaning publication Mother Jones that he took the first part of his dossier to the FBI "near the start of July."

James Comey, when he was FBI director, told members of the House Intelligence Committee the Trump-Russia investigation began "in late July."

So the timeline is: The first dossier report was June 20, Steele approached the FBI near the start of July, and the FBI began its investigation in late July.

Steele's first dossier installment, the June 20 document, cited a "senior Russian Foreign Ministry figure" and a "former top level Russian intelligence officer still active inside the Kremlin." It reported that "Russian authorities had been cultivating and supporting US Republican presidential candidate, Donald TRUMP for at least 5 years" and that "the TRUMP operation was both supported and directed by Russian President Vladimir PUTIN." Also citing a "senior Russian financial official" and a "close associate of TRUMP who had organized and managed his recent trips to Moscow," the dossier said Russians had been feeding Trump "valuable intelligence" on Clinton "for several years." The report also said "TRUMP's (perverted) conduct in Moscow included hiring the presidential suite of the Ritz Carlton hotel, where he knew President and Mrs. OBAMA (whom he hated) had stayed on one of their official trips to Russia, and defiling the bed where they had slept by employing a number of prostitutes to perform a 'golden showers' (urination) show in front of him." The action was all captured by hidden cameras, the dossier said.

The FBI was very interested in Steele's report, according to Mother Jones' David Corn, who was personally briefed by Steele:

The former intelligence officer says the response from the FBI was "shock and horror." The FBI, after receiving the first memo, did not immediately request additional material, according to the former intelligence officer and his American associates. Yet in August, they say, the FBI asked him for all the information in his possession, and for him to explain how the material had been gathered and to identify his sources. The former spy forwarded to the bureau several memos — some of which referred to members of Trump's inner circle. At that point, he continued to share information with the FBI. "It's quite clear there was or is a pretty substantial inquiry going on," he says.

Corn's report suggested the FBI was surprised by the dossier report's contents, which in turn suggested the FBI wasn't already on the case when Steele approached the bureau near the start of July.

Not long after, on July 7, Carter Page, whom Trump had named to a little-used foreign policy advisory board, began a three-day visit to Moscow, where he gave a public commencement speech to a university known as the New Economic School.

In a dossier report dated July 19, Steele wrote that Page had met in Moscow with Igor Sechin, the CEO of Rosneft, the giant Russian oil company, and, separately, with Igor Divyekin, a senior figure in the Putin government. Sechin was known to be very close to Putin and was also under U.S. sanctions. According to Steele's dossier report, Page and Sechin discussed ending U.S. sanctions against Russia. (A later dossier entry said Sechin offered Page billions of dollars to have Trump lift those sanctions.) Page and Divyekin allegedly discussed "a dossier of 'kompromat' the Kremlin possessed on TRUMP's Democratic presidential rival, Hillary CLINTON, and its possible release to the Republican's campaign team."

The dossier information made its way from Steele to the FBI to Capitol Hill. On August 27, the Senate's then-Minority Leader, Harry Reid, wrote a letter to Comey noting "a series of disturbing reports" about whether "a Trump advisor who has been highly critical of U.S. and European sanctions on Russia…met with high-ranking sanctioned individuals while in Moscow in July of 2016, well after Trump became the presumptive Republican nominee."

Steele continued to file dossier reports to the Clinton campaign and, apparently, to the FBI during the July-August-September-October time frame -- in other words, the period leading up to the November 8 presidential election. There were reports dated July 30; August 5, 10, and 22; September 14; and October 12, 18, 19, and 20.

The challenge -- for Steele and for Fusion GPS, the opposition research firm that hired him with funding from the Clinton campaign -- was to get the dossier's charges out in public, where they might influence the presidential race.

As the election approached, the Clinton campaign, through Fusion GPS, directed Steele to give the dossier information to a few journalists. At the "end of September," and again in October, according to British court papers, Steele personally briefed reporters from the New York Times, Washington Post, CNN, the New Yorker, and Yahoo.

The allegations in the dossier were basically impossible for journalists to verify. Most of the publications did not report the information. But on September 23, Yahoo's Michael Isikoff reported that "U.S. officials" had "received intelligence reports" from a "well-placed Western intelligence source" that Carter Page had met with Sechin and Divyekin during Page's July trip to Moscow. Isikoff wrote that some American officials were "taken aback" by the report that Page had met Sechin and Divyekin and were "seeking to determine" whether Page had "opened up private communications with senior Russian officials" on the sanctions issue. The allegations "have been discussed with senior members of Congress," Isikoff added, quoting from Reid's August 27 letter to Comey.

The Clinton campaign tried to publicize the report, but the dossier news did not really catch on. So in late October, according to those British court papers, with election day fast approaching and the allegations still not out, Fusion, acting on behalf of the Clinton campaign, directed Steele to brief Mother Jones. David Corn's article, which stayed away from the most incendiary parts of the dossier, was published on October 31, 2016.

One day earlier, on October 30, Harry Reid tried again to push the information into public view with a letter to Comey stating that the FBI possessed "explosive information about close ties and coordination between Donald Trump, his advisors, and the Russian government."

Of course, Comey knew what the FBI possessed. The point of Reid's letter was to get it out in public as the election approached. Again, there was some attention paid, but November 8 came and went without Americans excitedly discussing something called the Trump dossier.

Now, with multiple investigations underway, some officials are trying to reconstruct the events of June through October 2016. Were the allegations in the dossier accurate in the first place? If they were, did involvement go to the highest levels of the Trump campaign? But if they weren't, was it a situation in which the Clinton campaign, through its hired foreign agent Steele, fed the FBI bad information for the purpose of having it leaked to the press in time to hurt Trump before the election?

On the question of accuracy, it's just not clear what in the dossier has been verified and what hasn't. If one reads the dossier to be making the broad point that Russia tried to interfere in the 2016 election -- well, that's true. And indeed, some commentators have claimed that the dossier "checks out." But if one reads the dossier in all its damaging details and looks for confirmation -- well, that confirmation can be hard to find.

Page, for example, in testimony to the House Intelligence Committee on November 2, denied meeting with Sechin and Divyekin in Moscow, and it's not clear there is any evidence, beyond the dossier, that he did. Trump executive Michael Cohen has strongly denied the parts of the dossier about him, and it's not clear there is any evidence, beyond the dossier, to prove him wrong. And of course, on what is perhaps the dossier's key big-picture allegation -- "There was a well-developed conspiracy of cooperation between [the Trump campaign] and Russian leadership" -- the jury is still very, very out.

Still, there's one more important factor to be considered in assessing the dossier's role in the FBI investigation. According to papers released as part of his plea of guilty to lying to investigators, Trump volunteer advisor George Papadopoulos admitted having contacts with possible intermediaries to high-ranked Russians who are said to have offered assistance to the Trump campaign. That happened beginning in March, 2016 and continued for a few months. What is not known is whether the FBI knew about Papadopoulos' activities as they happened, or whether the bureau found out about them later, and in any event whether or not the Papadopoulos matter was, along with the dossier, part of the FBI's decision to start a counterintelligence investigation. It does seem clear that the Papadopoulos affair did not prompt the FBI to start a counterintelligence probe in March, or April, or May, or June of 2016.

What is absolutely clear is that, beyond any investigation, the dossier has proven enormously useful politically to the president's adversaries. At that House Intelligence Committee hearing on March 20, 2017, for example, at which Comey announced the existence of the investigation, Democrats spent a lot of time raising very specific allegations from the dossier. Carter Page's name was mentioned 28 times, and Sechin's 11 times. Comey said he could not discuss the allegations in public. But Democrats made the dossier a centerpiece of their case against Trump.

Now the moving parts, as far as the dossier is concerned, are the Mueller investigation and Congress. With Mueller, it's impossible to determine whether he is using the dossier and if so, in what way. For its part, Congress is trying to uncover the dossier story — what did the FBI do to try to verify it? Did agents use it as a basis for seeking wiretaps? — but getting information out of the FBI, as well as Fusion GPS, has been like pulling teeth, even after a House subpoena.

Ultimately more will become public. But if the past months have shown anything, it is that the FBI will not reveal its secrets, even to its legitimate congressional overseers, without a fight.