A false fire alarm led approximately 1,500 employees of the California Department of Transportation to evacuate the agency’s building Thursday at 11th and N streets in downtown Sacramento.

Mark Dinger, a Caltrans spokesman, said a fire alarm went off shortly after 1 p.m. in the department’s sixth-floor computer room. He said the six-story building was quickly evacuated, and crews from the Sacramento and West Sacramento fire departments responded.

No fire was found.

Dinger said a roofing contractor was working on the roof, and it is thought the fire alarm might have been triggered by particles of material that drifted into vents.

He said the evacuation lasted for about an hour.