Michael Flynn was once one of Trump's most trusted advisers. Now he could bring his administration crashing down

Admission of guilt puts Robert Mueller's probe right inside America's most famous home

Michael Flynn used to lead the raucous chorus of “lock her up” when Donald Trump berated “Crooked Hillary” in his aggressive, overheated campaign rallies. Now it is the former army general, once one of the most trusted of all the President’s men, who may start the process which can bring this administration crashing down.

Mr Flynn’s plea of guilty at a federal charge of lying to the FBI is more serious than the indictment of Mr Trump’s former campaign manager Paul Manafort. This is someone admitting to a felony who was no less than the National Security Advisor, privy to the most sensitive secrets, in a position to do grave damage to national security.

Mr Trump used to take Mr Flynn to the intelligence, defence and foreign policy briefings he received from the CIA, FBI and the Pentagon while running for president. And, as National Security Advisor, Mr Flynn would have direct access to the classified information when his man was in the White House. One key line of special counsel Robert Mueller’s investigation would be whether any of the material was compromised by clandestine contact with the Russians.

Crucially, the indictment of Mr Flynn puts former FBI director Mr Mueller’s investigation inside the White House, and not just on the campaign trail as had been the case with Mr Manafort. Very little has been released so far from the special counsel’s team about the charges. But the document does indicate that the investigation is focusing on how the former National Security Advisor’s liaison with the Russian ambassador, Sergei Kislyak, could have influenced presidential policy.

We now know, from Mr Mueller’s office, that Mr Flynn had talked to Mr Kislyak about Russia’s response to sanctions imposed by the outgoing Obama administration and the course of action the Kremlin may take at the United Nations. He has now admitted lying to the FBI about these conversations, indicating his apprehension about details coming out.

We also know that Mr Flynn was not flying solo in the Mr Trump camp in his dealings with Moscow. The prosecutors state that he called “a senior member of the presidential transition team” on 29 December last year after Mr Kislyak had contacted him. The message Mr Flynn was asked to pass on to the ambassador was that Moscow should not escalate the situation, with the obvious inference that they will receive something in return. The Russians duly obliged. Mr Trump tweeted the next day “Great move on delay (by V Putin). I always knew he was very smart.”

That was not the first contact between Mr Flynn and Mr Kislyak with the apparent connivance of the Mr Trump team. A week earlier, on 22 December, prosecutors say that “a very senior member of the presidential transition team” personally directed Mr Flynn to approach a number of countries, to delay or defeat a UN security council resolution on Israeli settlements. Mr Flynn called Mr Kislyak the same day. On this occasion the ambassador could not help, however, and the resolution passed with 14 votes in favour and the US abstaining.

All this is emerging with the background of Mr Trump and his team repeatedly denying that there had been any coordination with the Russians during and after the presidential campaign, and this denial has continued even as evidence began to surface of almost 20 meetings between the two sides.

Mr Flynn may be in a position to provide Mr Mueller with the ammunition needed to dismantle the position being taken by the Trump administration. There is now little doubt that a plea bargain deal has been reached. There was early notice of that last week when Mr Flynn’s legal team announced that it will no longer be cooperating with White House lawyers.

Mr Mueller’s decision to accept a guilty plea by Mr Flynn on a fairly low level “1001 violation” named after 18 USC 1001, the criminal statute for making false statements, strongly indicated that the former National Security Advisor will provide significant help for the investigation. Defendants do not tend to get much credit for incriminating lesser targets, and Mr Flynn’s past closeness to Mr Trump means that the potential targets for Mr Mueller are Mr Trump’s senior officials, members of his family, or the President himself.

Mr Flynn will not be the first to be flipped by Mr Mueller’s investigators. George Papadopoulos, a Trump campaign aide, is cooperating. Last month the highly respected former US attorney Preet Bharara said that Mr Manafort, too, may go down the same path. And Felix Sater, a former business partner of Mr Trump and an associate of Russian and American organised crime networks, is helping federal authorities with an alleged Kazakh money laundering plot in which a Trump property is involved.

Mr Trump had tried hard to save Mr Flynn. The new President asked FBI director James Comey to ease off on pursuing him, allegedly saying during a private dinner: “I hope you can see your way clear to letting this go, to letting Mr Flynn go. He is a good guy, I hope you can let this go.” Mr Comey’s refusal to do so and, in effect, abandon the Russia investigation, led to his firing.

Steve Bannon, Mr Trump’s former chief strategist, called the sacking of Mr Comey one of the “biggest mistakes in modern political history” as it directly led to the appointment of Mr Mueller with his extensive powers. Mr Trump now waits to see just how damaging this mistake will be for his presidency as the net begins to close in around him.