Washington DC - President Donald Trump hailed his assassinated ally Charlie Kirk as a "martyr for truth" on Wednesday while assailing the "radical left," arguing its rhetoric contributed to the death of the conservative activist.

"For years, those on the radical left have compared wonderful Americans like Charlie to Nazis and the world's worst mass murderers and criminals," Trump said in a video posted on his Truth Social platform.

"This kind of rhetoric is directly responsible for the terrorism that we're seeing in our country today, and it must stop right now," he said.

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The president then vowed to carry out a crackdown on those responsible.

"This is a dark moment for America," Trump said.

"My administration will find each and every one of those who contributed to this atrocity, and to other political violence, including the organizations that fund it and support it."

Kirk – who helped boost support for Trump among young voters – was fatally shot Wednesday while speaking at an event at Utah Valley University.

His killer remains at large.

Video from the scene showed him addressing a large crowd when the sound of a single shot rang out. Kirk appeared to collapse in his chair before the camera swiftly moved and sounds of panic erupted in the audience.

Investigators said they believed the single bullet had come from a campus rooftop, fired by someone dressed in black, in what appeared to be a targeted killing.