YouTube shooter practiced at gun range before attack

Disturbing evidence has been released showing the YouTube shooter practised at a gun range just hours before her savage attack on Wednesday (NZ time).

Police say Nasim Aghdam went to a firing range before travelling to the YouTube headquarters in San Bruno, California, and opening fire with a 9mm Smith & Wesson handgun.

Three people were hospitalised with gunshot wounds, and one remains in a serious condition. Aghdam was killed by a self-inflicted wound.

Aghdam's videos focused on animal cruelty issues, and she was reportedly motivated by anger because YouTube had restricted her videos.

"It is believed the suspect was upset with policies and practices of YouTube," San Bruno police chief Ed Barberini told reporters.

"This appears to have been the motive for this incident."

Law enforcement authorities are investigating a website that appears to list Aghdam's grievances against YouTube. They are also looking into an Instagram page that focuses on vegan issues.

"There is no equal growth opportunity on YOUTUBE or any other video sharing site, your channel will grow if they want to!!!!!" one post reads.

"YouTube filtered my channels to keep them from getting views!"

She also quotes Nazi leader Adolf Hitler, saying: "Make the lie big, make it simple, keep saying it, and eventually they will believe it."

Her family say they tried to warn police Aghdam was a threat. But Mountain View police say there wasn't enough evidence of danger.

"Her father called us back to let us know that she made a series of vegan videos for her channel on YouTube, and that the company had recently done something to her videos that had caused her to become upset," Mountain View Police said.

"Once again, at no point did her father or brother mention anything about potential acts of violence or a possibility of Aghdam lashing out as a result of her issues with her videos. They remained calm throughout this second phone call."