The rabbit hole just got much, much deeper for Donald Trump. After it was seemingly confirmed by last week's testimony from former FBI Director James Comey, it's now been made official this evening: Special Counsel Robert Mueller is investigating Trump for obstruction of justice, a felony.
The news is coming in from multiple sources this evening, including the Washington Post (link). The news is potentially devastating for Donald Trump, and it likely means that his narrow window for trying to get away with preemptively firing Mueller has now closed.
Mueller will be interviewing everyone involved, including the NSA Director and the Director of National Intelligence, as Trump is suspected of having tried and failed to lean on all of them in an effort to derail the FBI's investigation of the Russia scandal.
Notably, the WaPo report asserts that Donald Trump has been under investigation since shortly after he fired Comey. So while that confirms that Comey was being truthful when he told Trump he wasn't being investigated at the time, Comey's demise has now changed things dramatically.
The fact that Donald Trump is now under investigation for obstruction of justice is crucial, because that's the same charge that forced Richard Nixon out of the presidency. Trump hasn't yet had a public response to the news.
(Source)