Nasim Najafi Aghdam, 39, had a channel on the site where she posted videos bemoaning animal cruelty, promoting jewellery that appeared to be handmade and demonstrated exercise techniques.
Aghdam, who was born in Iran, posted comments in Persian and English accusing online video-sharing site YouTube of censorship.
“I am being discriminated. I am being filtered on YouTube,” she said in an English-language video that was posted to her YouTube account. The channel was deleted on Tuesday.
“We know that she was upset with YouTube … that’s the motivation,” San Bruno Police Chief Ed Barberini told the media.
“Whether that rises to the level of terrorism hopefully will be determined in the next couple of weeks,” he said.
The shooting at YouTube’s corporate headquarters in San Bruno came days after Aghdam was involved in a dispute with her family.
She reportedly left the family home and on Monday, her family reported her missing, police said.
Early on Tuesday morning, Mountain View police found Aghdam sleeping in her car, a few miles away from Alphabet Inc’s – which owns YouTube – headquarters.
Officers who spoke with her said she was “calm and cooperative”, did not mention YouTube and gave no indication that she would harm anyone.
Police added that officers spoke with her family twice after finding her. Initially, her father had said nothing about her posing a threat to anyone but later called back to say that she had posted vegan videos to YouTube and was angry about something that had been done to them.
In a few posts she put up prior to the attack, Aghdam referred to herself in heroic terms for surviving in a hostile world.
“I think I am doing a great job. I have never fallen in love and have never got married. I have no physical and psychological diseases. But I live on a planet that is full of injustice and diseases,” she wrote in Persian on her Instagram account.
Around lunchtime on Tuesday afternoon, Aghdam went to the YouTube offices where she pulled a Smith & Wesson 9mm semi-automatic handgun and opened fire before turning the gun on herself. She did not appear to target any particular victims.
Police say that she had visited a gun range in the area on Tuesday morning.
Of the three victims, two were released on Tuesday evening, while a third, a man in his 30s, remained in San Francisco General Hospital in a serious condition on Wednesday.
According to reports on Wednesday by Los Angeles television station KTLA and other media, Aghdam’s family released a statement expressing their sorrow over the shooting and support for the victims.
“Our family is in absolute shock and can’t make sense of what has happened yesterday,” KTLA reports the statement as saying.
YouTube said it would be “revisiting” the “incident in detail” and will increase security at its offices worldwide.
The site has long faced complaints from users about censorship but defended itself by saying that it aims to balance its mission of fostering free speech with providing an environment for users that is appropriate and lawful.