What did the president know and when did he know it?
Flynn, a former Defense Intelligence Agency director, only held his position as Trump's national security adviser for 24 days.
In a statement Saturday, the Justice Department inspector general's office said that its investigators are "reviewing allegations involving communications between certain individuals, and will report its findings regarding those allegations promptly upon completion of the review of them".
He added, "We'll see what happens".
"He has pled guilty to those lies", Trump tweeted Saturday "It is a shame because his actions during the transition were lawful".
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"...just couldn't resist commenting on Flynn", Shaub tweeted. "We're unbeatable", Trump said.
According to new court documents, Flynn was in contact with senior Trump transition team officials at Mar-a-Lago about conversations with then-Russian Ambassador Sergey Kislyak regarding USA sanctions on Russia.
Neither Mr Trump nor Mr Pence indicated concern then that the Federal Bureau of Investigation had not been told the true story.
"I suspect there is a lot less to the Flynn plea than meets the eye", said Ari Fleischer, former press secretary to President George W. Bush.
In at least one instance, prosecutors say Flynn was directed by a "very senior member" of the presidential transition team to discuss a United Nations resolution on whether to condemn Israel's building of settlements.
Dec 23: Mr Kislyak tells Flynn that Russian Federation will not vote against the resolution.
These contacts with Kislyak may have been ill-advised, supporters said, but were not in and of themselves illegal. "If it involved collusion to hack, then Trump is in big trouble". Flynn said he did not recall Kislyak telling him of a change in Russia's response to US -imposed economic sanctions the two previously discussed.
Jan 20, 2017: Mr Trump takes office and Flynn becomes national security adviser.
Mueller's team also figures to zero in on other aides close to Trump.