A passerby called 911 shortly before 11 p.m. to report smoke coming from the roof of the Taqueria La Mexicana restaurant in the 13900 block of Highway 99 in unincorporated Snohomish County, officials said.
Damage was estimated at $400,000.
The restaurant was closed at the time and no one was inside.
Smoke and flames were visible inside the restaurant when firefighters arrived, Snohomish County Fire District 1 spokeswoman Leslie Hynes said.
"Firefighters initially entered the restaurant to attack the flames," Hynes said. "As conditions became more hazardous, they had to withdraw and attack the fire from outside."
Firefighters from fire district 1, Lynnwood and Mukilteo had the fire under control within about 20 minutes. No one was injured.
Northbound lanes of Highway 99 were closed for about an hour between Highway 525 and Lincoln Way while the road was blocked by fire apparatus and hoses.
The Snohomish County Fire Marshal's office is heading up the arson investigation, Hynes said.
Investigators are asking anyone with information about the fire to contact the fire marshal's office at 425-388-3557 or call 1-800-55-ARSON.
There is a $10,000 reward from the Arson Alarm Foundation for information leading to the arrest and criminal conviction of whomever is responsible.
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