Los Angeles County Fire Department personnel responded to the fire at about 9:50 a.m. after an explosion was reported at a rim polishing company on the 530 block of Commercial Way.
Forty employees were inside at the time of the blast. Eleven people were hospitalized, most for smoke inhalation. Two had critical burns.
"Our cause investigators have determined that the fire started up in the ducting system, that this was the result of a buildup of lint and dust," said L.A. County Fire Inspector Scott Miller. "There were materials in there that did speed up the rate of the fire."
Inspectors wearing hazardous-materials-protection suits took samples of the water runoff at the scene.
Cal-OSHA is assessing whether hazards were present and if workers were trained, and if they were wearing proper protective equipment, and if there was an emergency plan.
It was not clear when the last time the duct system was cleaned out.
Authorities closed Cypress Street between Lambert Road and La Habra Boulevard in response to the incident.
The explosion and fire occurred at Gorilla's Polishing Corp., part of Gorilla's Chrome, which plates and polishes metal, particularly car wheels.