The FBI is helping Colorado police investigate possible threats of violence made against state Supreme Court justices who barred Donald Trump from a presidential primary ballot.
The Denver Police Department also said it was providing "extra patrols" around the homes of the justices.
On December 19, they ruled 4-3 that Trump should be disqualified under a little-known clause of the US Constitution's 14th Amendment for engaging in insurrection.
Two nights later, officers attended the home of one of the justices in response to a call for service that police later described as an apparent "hoax report", adding they were still investigating the incident.
Police said in a statement on Tuesday: "The Denver Police Department is currently investigating incidents directed at Colorado Supreme Court justices and will continue working with our local, state and federal law enforcement partners to thoroughly investigate any reports of threats or harassment."
The FBI added: "We will vigorously pursue investigations of any threat or use of violence committed by someone who uses extremist views to justify their actions regardless of motivation."
Neither the police nor the FBI commented on the nature or extent of the incidents under investigation.
Republican strategists have suggested the Colorado ballot ruling, likely headed for a US Supreme Court appeal, would spark a backlash among political conservatives by reinforcing the narrative that Trump is the victim of a partisan legal process.
NBC News and other media outlets have since reported the emergence of violent rhetoric on right-wing online forums from Trump supporters aimed at the four Colorado justices who sided against him.
The posts in question included messages calling for the justices' personal information to be publicly exposed, and an apparent reference to the judges that said: "All f**ing robed rats must f**ing hang."
Trump became the first candidate in US history deemed ineligible for the White House under a provision of the 14th Amendment prohibiting officials who engage in "insurrection or rebellion" against the US government from holding elected office.
The Colorado high court held that the insurrection clause applies to Trump because of the role he played in stoking the January 6, 2021, attack on the US Capitol by a mob of his supporters seeking to block Congress from certifying Democrat Joe Biden's victory over Trump in the 2020 presidential race.
The Colorado court, whose decision applies only to the state's March 5 Republican primary, said it would delay the effect of its ruling until at least January 4, 2024, to allow time for an appeal.
