Flames tore through the Kortek Technology Integration building on Sobrante Way and Central Expressway in Sunnyvale.
Firefighters had to cut their way into offices and break down doors to gain entry. It took over an hour to put the fire out.
Several offices were damaged. The thick black smoke could be seen for miles.
"It was a considerable fire," said Battalion Chief Jeffrey Plecque. "One of the things that made this more significant was that the fire did vent through the roof of the building, and that lets the flame and the smoke to exit out the top of the building."
Fire crews say the business was closed at the time and no one was inside.