MORAGA, Calif. -- Smoldering cigarette butts left in a plastic trash can on the balcony of a nurse staffing business in Moraga appear to have started a three-alarm fire that caused more than $1 million damage Tuesday evening, a Moraga-Orinda Fire District spokesman said Wednesday.

The fire was reported shortly after 6:50 p.m. in a building that housed the headquarters of Agostini Nurse Staffing in the 1100 block of Moraga Way, Fire Marshal Michael Mentink said.

Sixty fire personnel battled the blaze for nearly two hours before they declared it under control at about 8:40 p.m.

Fire investigators were able to trace the origin of the fire back to a balcony that served as a designated smoking area and found evidence to suggest that smoldering cigarette butts were to blame.

Once the fire started, it climbed up the exterior wall of the two-story building and moved into the attic, Mentink said.

When firefighters arrived, they found heavy smoke showing from the second floor and roof of the two-story building, Mentink said.

"This is a good reminder to people to be careful with smoking materials," Mentink said.

The fire district received mutual aid from the Contra Costa County Fire Protection District and the San Ramon Valley Fire Protection District.

The fire, which was deemed accidental, caused an estimated $1 million damage to the building and about $300,000 damage to the building's contents.

No injuries were reported.