Police in Colorado are investigating threats made against judges who ruled Donald Trump cannot be listed on the state's presidential primary ballot.
Denver Police said they was providing extra patrols around the homes of justices in the city.
The Denver FBI office said it was assisting local police.
Colorado's Supreme Court ruled last week that Mr Trump is ineligible under a US Constitution clause that bars candidates who engage in insurrection.
Mr Trump and his campaign criticised the 4-3 decision, calling it "deeply flawed", and his campaign has vowed to appeal to the US Supreme Court.
The ruling relied on Section 3 of the Constitution's 14th Amendment, which was enacted after the US Civil War to block Confederate secessionists from returning to power in the reunified country.
The provision states that any official who "engaged in insurrection or rebellion" is disqualified from holding federal office.
The majority on the Colorado court ruled that Mr Trump's actions during the Capitol riot on 6 January 2021 amounted to insurrection.
After news of the ruling came out, a slew of threats against the judges were posted online, according to Advance Democracy, a non-partisan research group that monitors pro-Trump networks.
Some of the posts included personal details, such as the addresses and phone numbers of the judges, the group said.
Threats to hang and shoot the judges were posted to X, formerly known as Twitter, as well as the messaging service Telegram, the Trump-owned Truth Social network and several other fringe websites. Some have been taken down, while some were still live on Tuesday.
Among calls for peaceful protests and political action, there were also explicit calls for violence.
One user wrote: "If you live in Colorado, do what the founding fathers ACTUALLY would want. Grab your rifles and some rope."
In a statement, Denver Police said it would "thoroughly investigate any reports of threats or harassment" but declined to provide further details, citing privacy concerns and the ongoing inquiries.
Daniel J Jones, Advance Democracy's president, warned that he has observed "significant violent language and threats being made against the Colorado justices and others perceived to be behind Colorado's Supreme Court ruling".
"The normalisation of this type of violent rhetoric is cause for significant concern and it's appropriate for law enforcement to respond with protective actions," he added.
Alongside threats and warnings of violence and "civil war", there were also unevidenced allegations that the extreme rhetoric was a "trap" or "false flag" designed to goad law-abiding Trump supporters into committing violence.
One user wrote "I know it's all a trap," but said they would still "cheer" if the justices were victims of targeted violence.
The Colorado case was brought by a liberal watchdog group and a collection of anti-Trump Republican and independent voters.
Mr Trump has until 4 January to appeal the Colorado ruling, and legal experts say that it will have a tough time withstanding the scrutiny of the conservative-leaning US Supreme Court.
Mr Trump's Republican challengers have rallied around him on the issue, and Democrats are concerned that the court ruling will further fuel the former president's insistence he is being unfairly targeted by the judicial system.
Trump himself posted dozens of times on Truth Social over the holiday weekend, criticising the Colorado decision and the many other legal cases against him.
