Police confirmed the woman wounded three before taking her own life about 1pm local time at the office complex in San Bruno, California.
Two authorities have since told The Associated Press the incident is being investigated as a domestic dispute.
Hospital officials say a 36-year-old man is in critical condition, a 32-year-old woman in serious condition and a 27-year-old woman in fair condition, with the wounded man believed to be the suspect's boyfriend.
A national expert on mass shootings research, Jaclyn Schildkraut, said it was very unusual for a woman to commit such an act, saying women made up for only four percent of suspects in mass shootings in the U.S. between 1966 and 2016.
However the incident may not be considered as a mass shooting, as the Gun Violence Archive defines a mass shooting as four or more people shot or killed, not including the shooter.
At a press conference shortly after the incident, San Mateo County Sheriff's Department confirmed the shooter was dead.
'At 12.46pm this afternoon, we received numerous 911 calls regarding gun shots at YouTube's campus.
'Police arrived at 12.48pm and immediately began a search for a possible shooter or shooters.
'Upon arrival, police encountered numerous employees fleeing. It was very chaotic.
'Officers encountered one gunshot victim...several minutes later while searching the building officers located a second gunshot wound victim that may have been self-inflicted.
'Two additional victims were located several minutes later at an adjacent business,' a spokesman said.
The identity of the shooter and her victims have no been released.
This story is developing...
