Even by the White House's disastrous standards, it’s been a bad week for Donald Trump. First, the news on Friday that Michael Flynn – Trump’s short-lived national security adviser – was now actively co-operating with special counsel Robert Mueller. Mueller charged Flynn with lying to the FBI. This was over Flynn's dealings last December with Russia’s ambassador to the United States, Sergey Kislyak.

According to Mueller’s two-page criminal indictment, Flynn discussed sanctions with Kislyak and then lied about these conversations afterwards to federal agents. Not only that, but Flynn discussed the matter in December 2016 with Moscow's envoy after first talking to a senior Trump official, widely believed to be Trump’s son in law Jared Kushner. These backdoor manoeuvres by the incoming Trump team took place the day that President Obama expelled Russian diplomats in protest at the Kremlin’s hacking of the US election.

We don’t have the transcripts of what exactly Flynn said (though the FBI was bugging Kislyak, and does.) We do know that Vladimir Putin took the unusual step following Flynn's call of not taking reciprocal action against US diplomats in Moscow. The suspicion is that Flynn may have offered reassurances to the Kremlin that the new Trump White House was minded to drop sanctions against Russia - Putin's chief foreign policy ask.

For now, Republicans in Congress and the Senate are in no mood to remove the president. Over the weekend his tax-cutting proposals won approval on Capitol Hill in a rare piece of good news for the administration. Still, the scandal is creeping closer to the White House. Kushner is now in the frame and looks like Mueller's next target. Did Kushner brief Flynn following conversations with Trump, and if so what did Trump tell Kushner?

Meanwhile, on Saturday, Trump tweeted that he fired his national security adviser because Flynn lied to the FBI. Trump can only have known this if he talked with Flynn. The tweet looked to many like an inadvertent confession by Trump, the king of self-sabotage, that he obstructed justice. The timeline is damning. In February, a day after Flynn quit, Trump sought out FBI chief James Comey and asked him privately to let the “Flynn thing go”. Comey didn’t. In May Trump sacked Comey to make the "Russia thing" go away, instead triggering Mueller's collusion investigation.

Trump’s lawyer John Down now claims he wrote the damning tweet. Few believe him – the tone, breezy manner, and exclamatory style (“There was nothing to hide!”) all suggest this was the president’s authentic voice. An agitated one: Trump’s unhappy state of mind and repeated rubbishing of the FBI suggests a terror of where Mueller’s investigation might head next.

The question now is what exactly Flynn will tell Mueller. Flynn was an embittered critic of President Obama’s, who met Trump for the first time in August 2015 in New York. This was a few weeks after Trump announced his candidacy. Flynn functioned as an informal foreign policy adviser. He knows a lot. Later Trump picked him for the top national security job despite warnings from US intelligence not to do so.

Its concerns about Flynn were several. As head of the Defense Intelligence Agency, Flynn’s behaviour was erratic, various sources told me as I was writing my book Collusion, on the story of Trump and Russia. Flynn was, they said, obsessed with Iran, incapable of “linear thought”, and had a penchant for stupid conspiracy theories. Beyond this was a worry that Moscow appeared to be cultivating Flynn for its own secret purposes.

In 2013, when he was head of the DIA, the Kremlin invited Flynn to visit Russia. Kislyak arranged the visit which included a tour of the ‘Aquarium’ - the top secret headquarters of the GRU, Russia’s powerful military intelligence agency. Flynn pointed out correctly that Washington approved the trip; Flynn gave a lecture to Moscow’s senior military spies on “leadership”.

Still, why did the GRU host Flynn in Moscow? Viktor Suvorov—a former GRU major who defected to the West, and now lives in England — described Flynn’s visit to me as “very strange.”

“Oh my God, I had to eat my tie,” he said, when he learned of Flynn’s Aquarium drop-in. Suvorov added: “There’s something fishy going on. Can you imagine a top Russian adviser being invited inside MI6 or to lecture at the CIA: ‘We don’t know about leadership. Please tell us’?” The GRU was checking Flynn out, Suvorov said. “Maybe the Russians have some kind of material on him, or have him under control,” he speculated.

Flynn vehemently rejects any accusation of treason. He says he is now cooperating fully with Mueller in the interests of his family and his country. Nevertheless, there are further unanswered questions about a second trip to Moscow made by Flynn in December 2015, soon after the joined the Trump campaign. This was for the tenth anniversary dinner of RT, the Kremlin’s English language propaganda channel.

The Kremlin’s favourite personalities were there. Julian Assange—stuck in the Ecuadorian embassy in London— appeared by satellite. RT anchor Sophie Shevardnadze interviewed Flynn on stage, in front of around a hundred guests. There were a few Putin-friendly questions. Flynn sat against a backdrop of the channel’s green logo. According to the dossier by Christopher Steele, the former British intelligence officer, the Kremlin funded Flynn's trip to Moscow as part of its wide-ranging "anti-Clinton operation". It was seen as "successful in terms of perceived outcomes," Steele wrote.

The organizers found a special place for Flynn. Right next to him was Vladimir Putin. (Speaking to the Washington Post’s Dana Priest, Flynn said he had nothing to do with the seating arrangement. He hadn’t asked to sit next to Putin. Flynn said they were introduced but didn’t chat. He did learn that Russia’s president had a dim view of Obama and that Putin had “no respect for the United States’ leadership.”)

Flynn declined to say how much he was earning from RT. The answer, it later transpired, was $33,750. The money was compensation from a foreign government. Flynn should have requested permission in advance from the Department of Defense to accept this cash, but he didn’t. He made two further fee-paying speeches in Washington on behalf of Russian interests.

The fact that Mueller has not indicted Flynn over his Moscow visits – or over undeclared lobbying on behalf of the Turkish government – suggests Flynn has an interesting story to tell. He can shed light on the inner workings of the Trump campaign in 2015 and 2016. And on its multiple dealings with Russians. We don't have an answer to the Watergate question yet but are moving closer: what did the president know, and when did he know it?

Luke Harding is a foreign correspondent for the Guardian. His book, Collusion: How Russia Helped Trump Win The White House is out now (Guardian Faber, £14.99)