The theory that President Trump is or has been a Russian asset is a popular one among his detractors. But for the first time, we’re learning that it’s something the FBI suspected strongly enough to dig into.

The Washington Post has confirmed that the FBI launched a counterintelligence inquiry into whether Trump was working for Russia shortly after Trump fired FBI Director James B. Comey in May 2017. The news was first reported by the New York Times.

Practically speaking, this may not mean a whole lot. Special counsel Robert S. Mueller III was appointed mere days later, meaning any evidence the FBI collected was likely limited. It was Mueller’s decision to continue the line of inquiry, and we don’t know whether he has. But practical concerns aside, it’s a shocking story: The nation’s leading law enforcement agency was looking into whether a sitting U.S. president was working for a hostile foreign nation. The decision was something the FBI reportedly struggled with for months, and it still has its detractors.

But what might have led to such an extraordinary step by the FBI? And what’s the state of the evidence?

Comey’s firing was obviously the tipping point. Investigators reportedly shed their previous reservations about the inquiry after Trump’s televised admission to NBC News’s Lester Holt that the Russia investigation was on his mind when he did it. Another red flag was Trump’s attempts to include a reference to the Russia investigation in Deputy Attorney General Rod J. Rosenstein’s letter justifying the firing.

We already know that these few days contained a central event in Mueller’s investigation into whether Trump obstructed justice, but the idea that it also warranted a counterintelligence inquiry is notable. It’s one thing to deliberately hamper the investigation; it’s another to suspect Trump might have done so on behalf of Russia. Were this to ever lead to any concrete conclusions, that Holt interview will apparently have been an extraordinary misstep by Trump, who has often seemed to blurt out unhelpful statements about his true motivations.

The other obvious one here is the British former intelligence officer Christopher Steele’s dossier. The dossier included a high-profile allegation that Russia had kompromat — or compromising material — on Trump thanks to supposedly salacious evidence it had about Trump’s pursuits in a Moscow hotel room. This allegation has never been proved and Putin has denied it (as if he would confirm it), but Comey has suggested it’s not out of the question, and some lawmakers have even gone so far as to raise the idea that Trump is compromised.

“Millions of Americans will continue to wonder if the only possible explanation for this dangerous and inexplicable behavior is the possibility — the very real possibility — that President Putin holds damaging information over President Trump,” Senate Minority Leader Charles E. Schumer (D-N.Y.) said on the Senate floor last year.

Added Sen. Jeff Merkley (D-Ore.): “I think it’s likely.” A retiring GOP congressman, Charlie Dent (Pa.), agreed.

The Times outlined a number of other events that played into the obstruction case and could have fed further suspicions about Trump’s motivations, including his pro-Russia and pro-Putin campaign-trail rhetoric and his public request that Russia try to obtain Hillary Clinton’s emails. The GOP also altered its platform on Ukraine in a more pro-Russia direction.

What hasn’t been outlined, though, are the proposed back channels between the Trump team and Russia.

A month before Comey was fired, The Washington Post reported that Trump ally and Blackwater founder Erik Prince had proposed a secret channel of communication between Trump and Moscow at a Jan. 2017 meeting in the Seychelles with a Putin representative. The FBI was also presumably aware at the time (because it monitors the calls of Russian officials on U.S. soil) that then-Russian Ambassador Sergey Kislyak had told his superiors in Moscow that Trump’s son-in-law, Jared Kushner, had proposed a back channel during the transition period. (The Post reported this shortly after Comey’s firing.)

If Trump was working for Russia, it would be logical to assume he’d need some way of actually learning what Russia desired — which would be difficult through regular channels, given they’d be monitored. We don’t know where these back channels stand in the Russia investigation, but if Mueller were probing a potential secret Trump-Russia alliance, you’d think they’d be of interest.

For similar reasons, Trump’s meeting with Putin in Helsinki last year has also raised eyebrows. He met privately with Putin for two hours, with nobody but interpreters present, and apparently nobody in the American government really knows what they discussed. Director of National Intelligence Daniel Coats even seemed to express concern about the lack of information. “I’m not in a position to either understand fully or talk about what happened in Helsinki,” Coats said afterward. Why Trump would need to keep things so under wraps has always been curious.

There are, of course, simpler explanations to all of this than the idea that Trump was working for Russia. Perhaps he truly admires Putin’s leadership style — which very much fits with his expressed admiration and work with other authoritarian leaders. It has been clear that Trump wanted to do business in Russia, so he seemed to be positively predisposed toward the country. And his efforts to hamper the Russia investigation needn’t be about any secret pro-Russia agenda; it’s also quite logical to think Trump simply views the whole thing as casting a pall over his election and raising concerns about the legitimacy of his presidency. Even if Trump unjustly attempted to obstruct the investigation, that doesn’t mean he was necessarily doing it for Russia. In fact, Russia would seem to have less to gain from such obstruction than Trump would.

But there was apparently enough subterfuge and concern here to cause the FBI to take an extraordinary step — even if it might have wound up being a brief one. The idea that Trump’s interests might not indeed be “America First” has largely bubbled beneath the surface of American politics for the past two-plus years. It’s still highly speculative, based upon the public evidence, but as always, the question is what Mueller knows that we don’t.