Authorities were investigating Wednesday how a second-alarm fire was sparked overnight at a Gardena tire shop. The fire was first reported at 2:21 a.m. at Collins Firestone, 1112 W. Gardena Blvd. in Gardena. Arriving firefighters found flames that threatened to spread to neighboring buildings and bring down the entire tire shop, according to Los Angeles County Fire officials. The blaze was knocked down within a half hour after the second alarm call and a partial collapse. No injuries were reported, and more than 80 firefighters at the scene kept the fire from spreading to two adjoining businesses. A damage estimate was not immediately available.