Russia probe: In Michael Flynn, Robert Mueller unearths more lies and a key witness

More smoke but no smoking gun.

Michael Flynn's guilty plea added a new layer of lies to the far-reaching investigation into ties between President Donald Trump and Russia, and put heightened scrutiny on the president's son-in-law, Jared Kushner.

But Flynn's admission, and all of the criminal cases thus far, have not resolved the fundamental question special counsel Robert Mueller is seeking to answer: Did Trump's campaign collude with Russia to win the election? Still, Mueller has left no doubt that his investigators have amassed a wealth of knowledge about the contacts between Trump associates and the Russians, and they're looking to gather more facts from Flynn, a new key cooperator.

By forcing Flynn's assistance, Mueller gains someone who can put him in the room with Trump and his closest advisers during the campaign, transition and the early days of the administration, times where Trump associates have acknowledged communicating with people connected to Russia.

In the hours after Flynn admitted lying about his contacts with a Russian government official, two names surfaced as integral players in his actions.

Kushner was identified as a "very senior" transition official, who directed Flynn to contact foreign governments, including Russia, about a UN Security Council resolution last December. And KT McFarland, who served as Flynn's deputy national security adviser, was a "senior" transition official involved in discussions with Flynn about what to relay to Sergey Kislyak, then Russia's ambassador to the US, about the response to US sanctions levied by the Obama administration.

Kushner and McFarland weren't named in court papers. But McFarland's involvement was confirmed by two former transition officials who spoke on condition of anonymity because they weren't authorised to publicly discuss the matter. One of the officials confirmed Kushner's involvement.

Flynn became the fourth person known to have been charged in Mueller's probe and the second, after former campaign policy adviser George Papadopoulos, to cooperate with investigators.

For both Flynn and Papadopoulos, prosecutors employed a similar, and textbook, strategy by accepting a limited guilty plea and turning the defendants into government cooperators.

Papadopoulos and Flynn both pleaded guilty to lying to the FBI about their foreign contacts but not for their underlying conduct.

Still, Flynn's plea to a single felony count of false statements made him the first official of the Trump White House to admit guilt so far in Mueller's criminal investigation as court papers made clear that senior Trump officials were aware of his outreach to Russian officials in the weeks before the inauguration.

That revelation moved the Russia investigation, which has overshadowed Trump's agenda throughout the year, deeper into the White House and raised questions about the accuracy of administration assertions that Flynn had misled Vice President Mike Pence and other officials about his calls with Kislyak.

Though prosecutors also had investigated Flynn lobbying work on behalf of the Turkish government, the fact he pleaded guilty to just one count, and faces a guideline range of zero to six months in prison, suggest prosecutors see him as a valuable tool and are granting a degree of leniency in exchange for his sharing what he knows.

Flynn, a 58-year-old retired US Army lieutenant general, accepted responsibility for his actions in a written statement: "My guilty plea and agreement to cooperate with the Special Counsel's Office reflect a decision I made in the best interests of my family and of our country." Immediately after Flynn's plea, White House lawyer Ty Cobb sought to put distance between Trump and the ex-aide, saying, "Nothing about the guilty plea or the charge implicates anyone other than Mr Flynn."

For his part, the president ignored reporters' shouted questions as he welcomed the Libyan prime minister to the White House, and aides cancelled media access to a later meeting between the two.

He did appear briefly at an afternoon White House holiday reception for the media, where he offered season's greetings and departed without addressing the Mueller investigation.

Trump grew close to Flynn during the campaign. The general was a vocal and reliable Trump surrogate, known for leading crowds in "Lock her up" chants regarding Democrat Hillary Clinton's use of a private email server. After his election victory, Trump elevated Flynn as his top national security adviser.

But Flynn's White House tenure was short-lived. He was forced to resign last February following news reports revealing that the Obama administration officials had informed the Trump White House that Flynn had discussed sanctions with Kislyak, a fact at odds with the public assertions of Pence.

The officials warned that the discrepancy made the administration potentially vulnerable to Russian blackmail.

After Flynn's departure from the White House, Trump retained a special interest in his former adviser.

Former FBI Director James Comey, whose firing in May precipitated the appointment of Mueller as special counsel, has said Trump asked him in a private Oval Office meeting to consider ending the investigation into Flynn. Comey has said he found the encounter so shocking that he prepared an internal memo about it.

That FBI investigation was the basis of the court case against Flynn, centring on a series of conversations that Flynn had with Kislyak during the transition period between the November election and the January 20 inauguration.

Prosecutors say Flynn on December 29 spoke with an unnamed senior transition team official about what, if anything, to say about sanctions that had been imposed on Russia one day earlier by the Obama administration in retaliation for election interference.

Flynn requested the Russian ambassador "not escalate the situation" and respond "in a reciprocal manner," a conversation prosecutors say he then reported to transition team members. Just days later, Vladimir Putin opted not to retaliate.

Another conversation with Kislyak occurred one week earlier after a "very senior member" of the presidential transition team directed Flynn to contact foreign government officials, including from Russia, about a UN Security Council resolution regarding Israeli settlements.

In a striking rupture with past practice, the Obama administration refrained from vetoing the condemnation of the settlement expansion, opting instead to abstain. The rest of the 15-nation council, including Russia, voted unanimously against Israel. At the time, Israel was lobbying furiously against the resolution and the Trump team spoke up on behalf of the Jewish state.

Former US officials and foreign diplomats said Kushner led the transition effort to defeat that UN vote.

During his conversation with Kislyak, prosecutors say, Flynn requested that Russia vote against or delay the resolution, though he admitted in his plea deal that he later lied to the FBI by saying he had not made that request.