The electrical fire was first reported around 3 a.m. at Building 150; the building is where research is done on ballistic missile defense systems and airborne lasers.
Firefighters approached the blaze with a lot of caution.
"There are certain buildings that carry hazardous materials that would be a greater risk to us, but this happened to be a building that was office spaces, so we did not have those concerns," said Sunnyvale Fire Department Capt. Carl Rushmeyer.
Firefighters said there were no missiles inside the building; it's mostly where paperwork is done. The building is in a secured location and houses top secret information.
Finding the fire was challenging to firefighters, because Lockheed's facilities are nearly 200 acres in Sunnyvale.
About seven fire engines remain at the scene.
There were employees who were evacuated.
Lockheed is the nation's largest missile defense contractor. The company is also involved in aerospace research.