After President Trump fired former FBI director James Comey in May 2017, the FBI opened an investigation into whether Trump was secretly "working on behalf of Russia against American interests," the New York Times reports.

Details: Intelligence officials had their concerns about Trump's ties to Russia during the 2016 campaign, but held off on opening an investigation because of the sensitive and potentially explosive implications that it came with. It was only after Trump twice tied the firing of Comey to his desire to put an end to the Russia investigation that officials decided to launch an inquiry into "whether the president’s own actions constituted a possible threat to national security," per the Times.

The first red flag that alerted officials to the possibility that an investigation was necessary was an early draft of Comey's firing letter, in which Trump thanked Comey for telling him he was not a target of the Russia probe. The second was when Trump told NBC's Lester Holt in an interview that he had fired Comey because of the Russia investigation.

FBI officials reportedly felt validated when it was later revealed that Trump told Russian officials in the Oval Office, "I faced great pressure because of Russia. That’s taken off,” just days after the Comey firing.

The investigation had both a counterintelligence and criminal aspect to it — with the criminal question being whether he had obstructed justice by firing Comey, per the Times. The probe has since been handed off to special counsel Robert Mueller.

The aggressive move to investigate Trump reportedly set off "a vigorous debate" within the Justice Department over whether FBI agents overreacted by opening the counterintelligence probe.

Statement from White House:

“This is absurd. James Comey was fired because he’s a disgraced partisan hack, and his Deputy Andrew McCabe, who was in charge at the time, is a known liar fired by the FBI. Unlike President Obama, who let Russia and other foreign adversaries push America around, President Trump has actually been tough on Russia.”

— Press Secretary Sarah Sanders

Go deeper: Timeline — Every big move in the Mueller investigation