The three-alarm fire broke out at about 3:15 a.m. at the plaza in the corner of La Mirada Boulevard and Leffingwell Road.
Fire officials said the roof partially collapsed during the firefight, which is why firefighters took a defensive stance in battling the blaze. One firefighter suffered minor burns but is expected to be OK.
Authorities believe the blaze started at a laundry business, traveled to the attic, and then spread quickly to other storefronts in the strip mall, engulfing four other businesses including a pet store. The pet store was said to be vacant.
"The attic is an open space and it spans throughout the entire building, and so once it gets up there, depending on the type of insulation, and the type of construction, if it's knob and tube, it can spread really quickly or ignite the insulation and spread at a really rapid rate," said Los Angeles County Fire Department Inspector Scott Miller.
Los Angeles County firefighters knocked down the blaze at about 5:30 a.m. Fire crews say they do not believe anyone was inside the building at the time of the fire.
The cause of the blaze is under investigation.