Fire officials Friday were investigating a suspicious blaze at Costa Mesa High School, the third such incident this month.
The fire gutted a classroom for hearing-impaired students at the school at 2650 Fairview Ave., said Connie Duddridge, risk manager for the Newport-Mesa Unified School District.
"Somebody's got a problem, and we're taking this very seriously," Duddridge said.
A janitor spotted flames about 8:30 p.m. Thursday and called for help. The building's brick walls helped contain the fire to one classroom. Although firefighters had the blaze under control within 13 minutes, it caused about $70,000 in damage, Battalion Chief Gregg Steward said.
On Friday, the class's 13 students were moved to another room while investigators sifted through charred desks, computers and books for clues. Also, an arson dog searched for liquids and chemicals.
"Three fires in three weeks, that's very suspicious," he said.
The two previous fires damaged portable toilets on the football field.