Witnesses said the woman wounded at least four people before taking her own life about 1pm (6am AEST) local time at the office complex in San Bruno, California.
“It was very chaotic, as you imagine,” San Bruno Police Chief told a press conference.
“We did encounter one victim with an apparent gunshot wound towards the front of the business as we arrived. A 37-year-old man is in a critical condition.
“Several minutes later while conducting a search of the premises, officers located a second individual with a gunshot wound that may have been self-inflicted.”