The FBI believes there were clear warning signs before the shooting of Charlie Kirk and are probing people in his orbit who may have had prior knowledge of it.

Investigators are delving into the transgender and gaming subcultures in Utah and scouring social media accounts after the discovery of a series of posts that appeared to reflect some awareness of suspected gunman Tyler Robinson and his alleged plot.

'There appear to have been multiple warning signs,' FBI assistant Director Dan Bongino said.

He said 'people in Robinson's network' were aware he had become 'infected' by far-left ideology and 'that his target was obviously going to be Charlie, and that people knew in advance.'

Bongino added: 'The question is...did they know? Were they sure of this? Or did they hear this and just write it off? That's what we're going to have to find out.'