The fire near the intersection of Collwood Avenue and East Valley Boulevard was reported at 5:11 p.m., according to the Los Angeles County Fire Department.
Firefighters went into defensive mode, pouring water on the flames from outside the structure, county fire Inspector Randall Wright said.
Dozens of firefighters were at the scene and Valley Boulevard was closed from the San Gabriel River (605) Freeway to Workman Mill Road, the fire department reported.
A civilian was treated at the scene for a burn injury, but there were no reports that anyone was taken to a hospital, according to fire department dispatchers.
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