Watch out President Trump because FBI Director James Comey keeps a "protective paper trail" says Matthew Miller, a former Department of Justice spokesperson under President Barack Obama.

A New York Times report on Thursday said Trump asked Comey to pledge loyalty to him in January, to which Comey declined.

In response to that report, Miller, who is vocal critic of Trump's, says to "stay tuned."

"One thing I learned at DOJ about Comey: he leaves a protective paper trail whenever something he deems inappropriate happened," he wrote.

Trump fired Comey on Tuesday after receiving a recommendation from the Justice Department to do so. However, Trump told NBC's Lester Holt on Thursday that he has long known that he was going to fire Comey, even before the recommendation.

Miller said in an opinion piece for the Washington Post that Trump "crossed a once-unthinkable red line" by firing Comey, whose agency is investigating Trump's campaign's ties to Russia.

Miller has also been critical of Comey, condemning him for repeatedly breaking Justice Department rules.