US investigators on Thursday released photos and a video of a person of interest in the fatal shooting of influential conservative activist Charlie Kirk and said they had found the rifle believed to have been used in the politically charged killing.

Kirk, a 31-year-old author, podcast host and close ally of US President Donald Trump, helped build the Republican Party’s support among younger voters. He waskilled on Wednesday by a single gunshot as he gave a talk at a university in Utah in what Trump called a “heinous assassination”.

Investigators have yet to publicly discuss any motive, but Trump told reporters that he had an indication of the killer’s motivation. “We’ll let you know about that later,” he said, adding that law enforcement was making “big progress” in the investigation.

FBI and state officials said the killer arrived on the campus a few minutes before the start of the event, a debate led by Kirk titled “Prove Me Wrong” outdoors in front of 3,000 people at Utah Valley University in Orem, Utah, about 65km (40 miles) south of Salt Lake City.

Security-camera videos show a person going up stairwells to get onto a roof before firing at Kirk, the officials told a press conference. Kirk, a staunch defender of gun rights, was answering an audience question about mass shootings when the bullet struck his neck. Audience members fled in panic.