A one story commercial building was on fire in El Monte around 11:30 a.m. on Saturday, authorities said.
The Los Angeles County Fire Department dispatched units to the scene in the 4600 block of Arden Drive at 10:36 a.m. to battle the blaze.
Around noon, the 2-alarm fire was upgraded to a 3-alarm fire.
"Crews are in defensive mode," the LAFD said in a tweet.
"There's a lot debris out there. They're (firefighters) hitting the hot spots," the dispatcher said around 1:45. "They have yet to announce a knockdown time. They're making progress toward a knockdown."
By 2:45, the fire was knocked down. Firefighters called for personnel from the Los Angeles County Division of Building and Safety as well as arson investigators, according to LACFD Dispatch Supervisor Bernard Peters, who described the damage as "significant."
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The commercial building was vacant, and no injuries were reported.
This is a developing story. Check back for updates.