Paul Manafort's in the hot seat. Who's next?

It's reported that, while he was chair of Donald Trump's presidential campaign, Paul Manafort offered to brief a Russian oligarch close to Vladimir Putin. That's just the latest revelation about evidence being collected by Special Counsel Robert Mueller as he probes Russia's effort to influence last year's election. Recent reports by CNN, the New York Times and the Washington Post suggest that Mueller's investigation has reached a "critical stage." Were the President's own associates involved? If so, did he know it? When he fired James Comey as head of the FBI, did he commit obstruction of justice? Trump has called the investigation a "witch hunt." We look at the "knowns" and the "un-knowns."