The woman identified by police as the attacker at YouTube’s headquarters in California was a vegan activist who shot three people because she was upset with the company’s policies and practices.

Police said 39-year-old Nasim Najafi Aghdam, from San Diego, was behind Tuesday’s shooting at YouTube’s offices in Silicon Valley, south of San Francisco, where the company, owned by Google, employs nearly 2000 people.

The attack left one man in critical condition and two women seriously wounded in the attack, which ended when Aghdam killed herself.

Aghdam accused the video-sharing service of discriminating against her, according to her online profile, while it has also been revealed that she practised at a shooting range only hours before the shooting and had no connection with anyone at YouTube.

“The San Bruno Police Department is investigating a motive for this shooting. At this time there is no evidence that the shooter knew the victims of this shooting or that individuals were specifically targeted,” police said in a statement.

Nasim Aghdam was 39. Photo: AAP

Aghdam’s online profile shows she was a vegan activist who ran a website called NasimeSabz.com, meaning “Green Breeze” in Persian, where she posted about Persian culture and veganism, as well as long passages critical of YouTube.

Aghdam, also often discussed animal cruelty; performed music parodies; and gave exercise tutorials on YouTube and on her personal website.

Police are also investigating a website reportedly linked to Aghdam which includes graphic photos of slaughtered animals and pleas not to wear fur jackets or have sex outside of marriage, according to a report in The New York Times.

The site also included her grievances against YouTube and Google for discriminating her, in English, Turkish and Persian.

“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!”

Another post accused “close-minded” YouTube employees of putting an age restriction on videos, claiming that the policy was designed to reduce views and to discourage her from from making new videos.

San Bruno Police Chief Ed Barberini told reporters on Wednesday: “We know that she was upset with YouTube … that’s the motivation, Whether that rises to the level of terrorism hopefully will be determined in the next couple of weeks.”

The shooting came days after Aghdam had a dispute with her family that caused her to leave her San Diego home. On Monday, they reported her missing, police said.

Early the next day, Mountain View police found Aghdam sleeping in her car a few miles from Alphabet’s headquarters. She was “calm and co-operative” with the officers she spoke with, made no mention of YouTube and gave no indication she would harm anyone, Mountain View police said.

The San Jose Mercury News quoted Aghdam’s father, Ismail Aghdam, as saying he told police that his daughter might go to YouTube’s headquarters because she hated the company but Mountain View police disputed that account.

Police said officers had twice spoken with the family after finding her. In an initial call, Aghdam’s father said nothing about his daughter posing a threat but then called back to say she had posted vegan videos to YouTube and was angry about something that had been done to them, police said.

“At no point did her father or brother mention anything about potential acts of violence,” the statement said.

While there are hundreds of mass shootings every year in the United States, mass shootings by women are rare.

The NYT reports that a 2014 FBI study of 160 shooting incidents found that only six involved a woman, and only three incidents between 2014 and 2016.

The most recent incident was the 2015 attack in San Bernardino, California, when Tashfeen Malik and her husband Syed Rizwan Farook attacked and killed 14, and wounded many more, of Farook’s colleagues at a holiday party.

A national debate around tighter curbs on gun ownership has intensified since the massacre of 17 people at a Florida high school in February shocked the public.

-with AAP