Frank Figliuzzi, the former assistant director for counterintelligence at the FBI, said he believed Mueller was set to issue indictments in the investigation of alleged Russian interference in the 2016 election, which could indicate what he might have on the president.
Tuesday marks the former anchor's return to TV since he was suspended in February.
The newsman admitted to wanting to "appear better" than he was.
Brian Williams was suspended without pay in February for exaggerating his experiencse.
How much do you need to trust the man reading a teleprompter?
Brian Williams decides to step aside for several days amid a probe of his reporting.