As far as we know, Trump was not personally under investigation when he fired former FBI director James Comey, but firing the FBI director has a way of changing things.
The Washington Post first reported last night that special prosecutor and former FBI director Robert Mueller is investigating Trump for obstruction of justice. And there's more.
Investigators have also been looking for any evidence of possible financial crimes among Trump associates, officials said. [...]
The obstruction-of-justice investigation of the president began days after Comey was fired on May 9, according to people familiar with the matter. Mueller’s office has taken up that work, and the preliminary interviews scheduled with intelligence officials indicate that his team is actively pursuing potential witnesses inside and outside the government.
According to the New York Times, the "financial crimes" Mueller is investigating includes possible money laundering.
A former senior official said Mr. Mueller’s investigation was looking at money laundering by Trump associates. The suspicion is that any cooperation with Russian officials would most likely have been in exchange for some kind of financial payoff, and that there would have been an effort to hide the payments, probably by routing them through offshore banking centers.
The Times report does not speculate who may have run Trump's money laundering operation, but I think we know who it was. We learned months ago that Trump's former campaign manager Paul Manafort, who has deep ties to Russian and Ukrainian oligarchs, was funneling money through offshore bank accounts in Cyprus. It's possible he was conducting legitimate business, but color me skeptical.
Trump had a meltdown on Twitter this morning where he accused the "deep state" of conducting a "witch hunt," but Trump has only himself to blame. He has personally made it very clear that he fired James Comey because he wouldn't drop the investigation of his buddy, former national security adviser Michael Flynn. Moreover, he simply won't keep his stupid mouth shut.