San Bruno police responded to shots fired at YouTube headquarters Tuesday afternoon, shortly after 1 p.m.
Details are still emerging, but initial reports indicate at least two people were injured.
Witnesses in the area said they saw dozens of firefighters and police officers descending on the area. Other witnesses said they saw police holding rifles and other weapons.
Producers from YouTube tweeted that they believed there was an active shooter on the company's campus. Some said they heard so many people running that they believed an earthquake had struck. Others described a gruesome scene.
"I looked down and saw blood drips on the floor and stairs," Todd Sherman, a YouTube producer, wrote on Twitter. "Peaked around for threats and then we headed downstairs and out the front."