Ashes from a barbecue grill discarded in a Dumpster accidentally set a Catholic high school in Mountain View ablaze and caused thousands of dollars in damage early this morning, a Mountain View Fire Department spokesman said today.

A passerby reported the two-alarm blaze at St. Francis High School at 1:21 a.m., fire spokesman Lynn Brown said.

He said several people had a barbecue at the school on Friday night during a football game, and that some people returned Monday for a Labor Day roast. They discarded the still-smoldering ashes into a Dumpster standing next to a loading dock for the school's kitchen.

The charcoal ashes, which Brown said can smolder for days unless properly disposed of in a metal container or soaked with water, flared up and reignited cardboard dumped in the trash container. The flames licked the wall of the kitchen and cafeteria while spreading upward toward the roof and attic.

Brown said firefighters had to pull down several ceilings to extinguish the hot attic fire, which damaged the school's heating and ventilation system.

Brown said several classrooms were unusable today but that construction crews were hard at work to restore power to the classrooms by Wednesday, when everything should be back to normal.

Graffiti reading "The Da Vinci Code" drawn on a school building appeared to be unrelated to the fire, and someone could have written the graffiti on Friday, when a lot of people were roaming the campus, Brown said.

The people who discarded the ashes were unlikely to face any penalties, such as fines for negligence, because the incident was an accident, Brown said.

He did, however, urge people to use barbecue grills with caution, noting that ashes should always be soaked with water and disposed of in metal containers away from flammable materials.