The blaze was reported at 7:20 p.m. Monday at 11070 Magnolia Ave., near La Sierra Avenue, on the city’s west side.
Riverside Fire Department Battalion Chief Garrett Coryell said nearly two dozen firefighters arrived to find a “working structure fire in the front office” of one of the businesses.
The fire was fully contained within 20 minutes to the room of origin and was traced to a failed electrical component, he said.
The building’s anti-fire sprinkler system activated during the blaze, causing some water damage, Coryell said.
He said one crew member suffered unspecified minor injuries but did not require hospitalization.