Before special counsel Robert Mueller began his investigation into the Trump campaign, the FBI opened a counterintelligence inquiry to determine whether Trump was working against America's interests on Russia's behalf, according to The New York Times.
The investigation was reportedly spurred by Trump's May 2017 firing of former FBI Director James Comey, and subsequent comments by Trump connecting the firing to the Russia investigation.
Timeline of events: Trump fired Comey on May 9, 2017. Mueller was appointed special counsel and opened his investigation on May 17. The FBI's investigation started sometime between those two events, and Mueller took over once he was appointed.
Why is noteworthy? The counterintelligence aspect of the FBI's investigation is noteworthy. It has long been known that Mueller and the FBI were looking into the potential of criminal obstruction of justice. But a counterintelligence inquiry would explore national security threats.
In short, the FBI was looking into "whether the president's own actions constituted a possible threat to national security."
It's not clear whether Mueller's investigation continued the counterintelligence aspect of the FBI investigation. The New York Times reported that the decision to open such an inquiry was controversial, with some in the Department of Justice believing that FBI agents had overreacted.
What made the FBI investigate: After Comey's firing, two comments by Trump raised red flags for the FBI. An early draft of Comey's termination letter reportedly included Trump thanking Comey for telling him he was not a target of the Russia investigation. And Trump told NBC that he fired Comey because of the Russia investigation.
The New York Times report acknowledged that "no evidence has emerged publicly that Mr. Trump was secretly in contact with or took direction from Russian government officials."