Michael T. Flynn, who was fired after just 24 days on the job, was one of the first Trump associates to come under scrutiny in the federal probe now led by Special Counsel Robert Mueller into possible collusion between Moscow and the Trump campaign.

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EXCLUSIVE: Mueller Has Enough Evidence to Bring Charges in Flynn Investigation 0:57

Mueller is applying renewed pressure on Flynn following his indictment of Trump campaign chairman Paul Manafort, three sources familiar with the investigation told NBC News.

The investigators are speaking to multiple witnesses in coming days to gain more information surrounding Flynn's lobbying work, including whether he laundered money or lied to federal agents about his overseas contacts, according to three sources familiar with the investigation.