"The death toll from the natural gas line explosion that leveled a San Bruno neighborhood grew overnight to four as federal investigators joined the probe into what caused the disaster. The morning after the inferno erupted, newly homeless residents of the Crestmoor neighborhood west of Interstate 280 awoke in shelters as firefighters and emergency officials took stock of the tragedy. The fire that raced through the neighborhood of single-family homes erupted at about 6:15 p.m. Thursday when a 30-inch Pacific Gas and Electric Co. natural gas pipeline ruptured, creating a 30-foot-wide crater, officials and the utility said. PG&E President Chris Johns said at a news conference this morning that the utility's workers had not yet determined why the pipe ruptured. Crews were seen near the edge of the crater by mid-morning. More than 12 hours after the explosion, the fire was still only 75 percent contained. Officials said 38 homes had been destroyed, down from an estimate Thursday night of 53, and seven more houses had sustained significant damage. Several other homes sustained less serious damage. The San Mateo County coroner's office said four people had been killed. Their identities haven't been confirmed pending comparisons to dental records, authorities said. By noon, coroner's investigators and emergency responders had searched 75 percent of the affected homes for victims, without finding anyone. The remaining homes were still too hot to approach. No one is known to be missing, authorities said. At least 52 people suffered burns, smoke inhalation and other injuries, and three of them were in critical condition with third-degree burns, authorities said. The most seriously injured were being treated at St. Francis Memorial Hospital in San Francisco. Three victims there have burns over 50 percent of their body, and a fourth has burns over 40 percent, said Dr. Michael Kulick. "