A 17-year-old boy was killed after a high-powered, illegal firework exploded in a backyard — rocking a neighborhood with a blast so powerful residents thought it was a bomb. The explosives detonated around 9:30 p.m. at a home on the 400 block of West 84th Street in the Vermont Knolls neighborhood of South LA late Wednesday , according to the LAPD. First responders found the boy in critical condition, and he was pronounced dead shortly after arriving at a local hospital.