YouTube shooter Nasim Aghdam was stopped in a California parking lot at 1:40 a.m. the day of the attack. She had been reported missing.
Police have now published a detailed account of the exchange.
They say Aghdam was "calm and cooperative" and gave them a plausible explanation.
She specifically said she had no plans to harm herself or others, police said.
Eleven hours later she opened fire on YouTube headquarters in San Bruno.
Police who found the YouTube shooter asleep in her car in a California parking lot on the morning of the attack said she seemed "calm" and told them she had no plans to hurt anybody.
Officers in Mountain View, California, matched Nasim Aghdam's license plates to a missing person report after they found her sleeping in a parking lot near a Walmart at 1:40 a.m. on the day of the shooting.
In a statement released after the shooting, the Mountain View Police Department published a long account of their interaction.
Its said officers spoke to her, asked why she had left home and, crucially, whether she intended to hurt anybody.
Police said she told them she had gone missing because of trouble at home, and was intending to stay with family nearby.
Here's how officers described the interaction:
"During our contact with her, she was asked a series of questions including, but not limited to, if she was a danger to herself or others.
"At no point during our roughly 20 minute interaction with her did she mention anything about YouTube, if she was upset with them, or that she had planned to harm herself or others.
"Throughout our entire interaction with her, she was calm and cooperative.
"At the conclusion of our interaction with her, she in no way met any reason for us to speak with her further or possibly detain her."
The statement was released after speculation that officers may have known Aghdam had designs on YouTube's campus, but missed the opportunity to detain her.
Aghdam's family said they were aware she had been angry with the video platform, and warned local police in San Diego that she might be heading to their San Bruno headquarters.
Police say they didn't know this when they met Aghdam. They said they spoke on the phone with her family, who initially made no mention of YouTube.
In a later call, Aghdam's relatives said she was "upset" with YouTube because of the performance of vegan-themed videos she had created, but did not seem panicked or suggest she had any violent intentions.
According to the New York Times, Aghdam spent the rest of the night in her car, bought a handgun later that morning, then visited a gun range to practice.
At around lunchtime she arrived at YouTube's headquarters and opened fire.