Comey’s firing on May 9, 2017, was not the start of an investigation of Trump. It was the point when the FBI and Justice Department rashly determined that they finally had a crime to pin on Trump — obstruction. In their haste and overconfidence, they rationalized that (a) Comey’s firing must have been intended to impede the Russia investigation, and that they could couple this with; (b) the claim that Trump may have impeded the Flynn investigation – based on a memo Comey leaked to the New York Times a few days after his firing.
Legally, none of this was obstruction. Yet, the FBI and Justice Department settled on this novel and flawed legal theory: Even though the president has constitutional authority to fire subordinates and weigh in on investigations, he may somehow still be prosecuted for obstruction if a prosecutor concludes that his motive was improper. Of course, even though he could have, Trump never actually took any steps to interfere in the investigations of Russia (which is still continuing) or Flynn (who later was indicted and pled guilty). Yet the FBI, hot-headed over the director’s dismissal, concluded that this obstruction theory was a sound enough basis to go overt with the case on Trump they had actually been trying to make for many months.
Remember, it is not just that the FBI formally opened an investigative file on Trump. There was talk between Deputy Attorney General Rod Rosenstein and acting FBI director Andrew McCabe of wiring up against the president — i.e., conducting covert surveillance to try to record Trump making damaging statements.