Shooter was wearing body armor.

[RandySF]

The building is being evacuated. There have been no confirmed reports of injuries or fatalities, but workers who were in the building described a chaotic scene as people scrambled away from a gunman wearing body armor.

Salahoden Abdul-Kafi, a YouTube product manager wrote on Facebook. “I was in the courtyard and we heard the gunshots then saw him. He had a shooting mask on, full body armor and was calmly walking and firing a handgun. We jumped to the floor then ran as fast as we could. I'm on my way home now.”

Abdul-Kafi said he was OK “but I don't know about a lot of coworkers.”

Todd Sherman, who identified himself as a YouTube product manager on his Twitter page, said he was sitting in a meeting and then heard that people were running. “First thought was earthquake.”


https://www.sfchronicle.com/news/article/Police-respond-to-reports-of-possible-shooting-at-12802771.php

[herding cats]

1. More than one shooter or is abc wrong?

Another law enforcement source told ABC7 News reporter Dan Noyes that the shooter is a white, adult female wearing a dark top and head scarf. A second law enforcement official told reporter Vic Lee the shooter is a woman.
http://abc7news.com/watch-live-shooting-investigated-at-youtube-headquarters-in-san-bruno/3297411/

What's a shooting mask?

[The Velveteen Ocelot]

2. They are reporting on MSNBC that the shooter was a woman (now deceased),

but if she was wearing body armor maybe her gender was not immediately apparent and assumed to be male because they usually are.