The surprise recommendation came ahead of Flynn’s upcoming sentencing, which had been postponed four times over the past year.

Those postponements indicated that, once hostile to the investigation that threatens Trump and his inner circle, he had possibly become a valuable witness.

“Given the defendant’s substantial assistance and other considerations set forth below, a sentence at the low end of the guideline range – including a sentence that does not impose a term of incarceration – is appropriate and warranted,” Mueller said in a memorandum to the court.

Flynn’s was the first guilty plea secured by the Mueller investigation into alleged collusion between Trump’s election campaign and Russia.

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His position as a top-level insider in the campaign, accompanying Trump to key events, making a keynote speech at the July 2016 Republican convention, and then taking hold of the White House national security apparatus in Trump’s first weeks in office, made him a potentially extremely valuable witness.

But many thought his own problems had likely compromised that value.

In an interview with investigators on January 24, 2017, four days after Trump’s inauguration, Flynn lied about conversations he had the previous December with Russia’s ambassador to the US, Sergei Kislyak.

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In those conversations, apparently recorded by US intelligence, Flynn appeared to be trying to undermine the policy of then-President Barack Obama by making separate political deals with Moscow.

Obama at the time was planning sanctions on Russia for its interference in the 2016 election. Flynn urged the Russians not to retaliate, suggesting Trump would reverse the sanctions.

Within weeks, Flynn was forced to resign after it was alleged that he also had lied to top White House officials about his Kislyak talks.

Then in March 2018, in a second interview with the FBI, Flynn lied about the fact that before and after the election, he had a US$530,000 lobbying contract on behalf of Turkey that he had not reported.

“At the time, the defendant was a national security adviser and surrogate for the Trump campaign who opined publicly on foreign policy and national security issues,” the sentencing memorandum noted.

Despite the light sentence recommendation, the memorandum gave no hint as to what Flynn had told the Mueller team about the operations of the Trump campaign and its Russia ties.

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Mueller has identified numerous contacts, including efforts by Trump aides during 2016 to set up a meeting between him and Russian President Vladimir Putin.

In another surprise, last week Trump’s former personal lawyer Michael Cohen admitted that through the first half of 2016, even after Trump had secured the Republican nomination for president, he had top-level contacts with Moscow over a project to build a potentially 100-storey Trump Tower in the Russian capital.

Mueller is also investigating a June 2016 meeting that top campaign staff, including chairman Paul Manafort, Trump’s son Donald Jnr, and son-in-law Jared Kushner, had with a Russian lawyer who had offered dirt on Trump’s election opponent Hillary Clinton.

Manafort breached his plea deal by lying repeatedly to the FBI and Mueller’s team, prosecutors alleged in a court filing last week.

And a separate document Tuesday suggested that Mueller has evidence that a Trump campaign adviser, Roger Stone, sought to coordinate with WikiLeaks as it published embarrassing Clinton-related emails that were stolen and fed to it by Russian intelligence.

A heavily redacted addendum to the Flynn memorandum Tuesday added more tantalising hints. It indicated that Flynn was also aiding one or more other federal criminal investigations related to the Trump campaign, but not being led by Mueller.

Those potentially could include an examination of Trump’s financial ties to Russia through his real estate business.

Adam Schiff, the top Democrat on the House of Representatives Intelligence Committee, which is also investigating allegations of Russian collusion, said the memo indicated Flynn had provided a significant boost to Mueller’s probe.

“That most of the details are redacted signals he has given far more than we or the President may know,” Schiff said on Twitter.

Trump has called Mueller’s probe a witch hunt and has denied colluding with Russia. Moscow denies meddling in the 2016 elections.

Additional reporting by Associated Press and Reuters