One investigation leader noted every new witness "puts about three more weeks into an investigation."
Rep. Elijah Cummings said the whistleblower came to him in June.
K.T. McFarland is President Trump's nominee for U.S. ambassador to Singapore. She's on hold over questions about Russia.
John Dowd told Axios that the Constitution guarantees President Donald Trump can't obstruct justice because he's the chief law enforcement officer.
The news of the agent's reassignment was first reported in August, but details were unclear.
A tweet from the president's account raised questions on whether he'd obstructed justice. But Trump's lawyer says he was the one who wrote the tweet.
Flynn must turn over any evidence of crimes he's aware of, take part in interviews and testify at relevant court proceedings.
Reports say sometime in November, Robert Mueller's team interviewed the president's son-in-law and adviser as part of the team's Russia investigation.
Lawyers with former national security adviser Michael Flynn reportedly will stop working with the president's legal team on the Mueller investigation.
Former national security adviser Michael Flynn allegedly plotted to abduct a Turkish cleric, according to a Wall Street Journal report.
CNN reports anyone charged could be taken into custody as soon as Monday.
Michael Flynn's family says he needs help footing legal defense bills that could reach $1 million.