FLORENCE, LOS ANGELES (KABC) -- Authorities believe illegal fireworks sparked a second-alarm fire that broke out at a mattress manufacturing building in Florence on Saturday.The fire was reported at about 9:40 p.m. in the 1600 block of East 76th Street.Some mattresses that were stacked outside near the rear of the building caught fire, which spread to the inside and several nearby buildings. One of the buildings was damaged beyond repair and will need to be torn down, authorities said.Witnesses told investigators that a group of males, possibly juveniles, were seen shooting illegal fireworks towards the mattresses. Authorities say the remnants of those fireworks were found all over the floor near the buildings.The other businesses affected included a silk screen company and a custom furniture shop.Los Angeles County firefighters knocked down the blaze and remained overnight to mop up hot spots.Four firefighters were injured and the fire caused between $1 million to $3 million in damage.The cause of the fire is under investigation.