SAN BRUNO, California — Fire crews flooded the ruins of burning homes with water early Friday and after a massive explosion apparently triggered by a broken gas line sent flames roaring through a neighborhood near San Francisco, killing at least four people and destroying more than 50 houses.Thursday night's explosion was heard for miles and shot a fireball more than 1,000 feet (300 meters) in the air, sending frightened residents fleeing for safety and rushing to get belongings out of burning homes, witnesses said.Utility officials said a natural gas line ruptured in the vicinity of the blast, which left a giant crater and sent flames tearing across several suburban blocks in San Bruno just after 6 p.m.