SAN BRUNO, Calif. — For weeks, residents in this community of trim suburban homes in the hills near San Francisco International Airport had reported catching the occasional whiff of natural gas in the bay breezes. Utility repair crews were regularly seen driving around the neighborhood.
And on Thursday night, just around dinner, a 30-inch natural gas pipe running three feet under ground erupted and fueled a devastating explosion and towering walls of wind-whipped fire, killing at least four people and consuming dozens of homes with a blaze that moved so quickly that residents barely had time to gather their belongings and run.
Throughout the day on Friday, firefighters struggled to put out the remnants of the blaze, search parties with dogs hunted for more bodies and residents huddled in Red Cross shelters, confronting the loss of their homes and the realization that part of their neighborhood had been reduced to a deep crater filled with water.
“I need to know if my house burned down,” Steve Hoff, 38, implored a California Highway Patrol officer at a police barrier that cordoned off the 15-acre disaster site on Friday. “I don’t know if I have a home left or not.”
The National Transportation Safety Board, which also investigates natural gas explosions, sent a team of four investigators to this city 12 miles south of San Francisco to determine what had caused the blast. One of the board members, Christopher A. Hart, toured the site on Friday afternoon and said he had been stunned by the destruction he saw. He said a large portion of the pipe had been blown out of the ground and across the road. “My immediate assessment was the amazing destruction, the charred trees, the melted and charred cars, the houses disappeared,” he said.
Officials at Pacific Gas and Electric promised to cooperate, but declined to confirm reports from many neighbors of a history of complaints of gas odors, or that a Pacific Gas repair truck had been spotted the other day near the area where the blast occurred.
“We have yet to be able to get close enough to the actual source to be able to determine exactly why this happened,” said Christopher P. Johns, president of PG&E. “We’re working diligently to do that.”
Mr. Johns said that the company was aware that residents were saying they had called complaining of gas odors and that the company was now going through records to see what calls were made and what the company’s response was. PG&E also said it would provide temporary housing, food and clothing for the survivors.
Christina Veraflor, 41, said she had visited her mother, who lives here, six weeks ago and noticed the smell of gas in the air. “That happened a lot in that area: You would get a whiff of gas, and then it would disappear.”
Darlene Esola, 59, a teacher who has lived in San Bruno for 26 years, said she noticed an odd odor while walking in the neighborhood on Tuesday night. “It smelled a bit gaslike,” Ms. Esola said, as she spent the afternoon at a Red Cross shelter where people waited to talk to insurance company representatives and passers-by dropped off food and clothing.
She said she had mentioned it to her husband but did not think of it again until after the explosion, when several neighbors also mentioned smelling gas in recent weeks.
Jerry Hill, a member of the State Assembly who represents the area, estimated that the pipeline had been installed in 1948, a reflection of just how old and worn much of the gas system is in this part of the state. He criticized the company for the way it had dealt with maintenance questions and responded to the crisis.
“If the indication is that there is a problem, a pipe deterioration problem or a gas leak problem, this needs to be resolved quickly and statewide,” Mr. Hill said.
The utility has some history of problems with federal regulators, records show. The N.T.S.B. cited PG&E for shortcomings in its response to a 2008 natural gas leak in Rancho Cordova, one that eventually killed a resident and injured two more in an explosion.
The N.T.S.B. found that the company had used improper piping that allowed gas to leak from a mechanical coupling in 2006. When a neighbor smelled gas, the company delayed nearly three hours before sending a properly trained crew to identify the leak, and the slow response was a key reason for the explosion that same day, according to the N.T.S.B. report.
The 2008 episode had been the worst of 65 gas pipeline accidents involving PG&E since 2004; cases that include nine other injuries, according to federal Department of Transportation records. Seventeen incidents were deemed “significant” by federal regulators because large amounts of gas escaped from leaking pipes, ruptures and other failures.
More than half the time in all the accidents, the gas either ignited or led to an explosion. At least 16 episodes also spurred evacuations of dozens of homes and businesses, in part because of threats to public safety.
Mr. Hart, the member of the N.T.S.B., said it could take up to 18 months until the board issued its finding on what caused the blast, but he said that if it discovered any safety lapses, it would not delay announcing them and recommending changes.
Lt. Gov. Abel Maldonado, who was running this state while Gov. Arnold Schwarzenegger was on a trade mission to China, said the authorities had gone through 75 percent of the disaster site. The rest, he said, was too hot to confront.
As of late Friday, besides the 4 people confirmed dead, 52 people were injured and 3 suffered third-degree burns, Mr. Maldonado said. A total of 37 structures had burned, and 7 more were damaged, he said. There was one arrest on Thursday night for looting. As many as 400 firefighters from local and state crews responded to the blaze, which began about 6:30 p.m. Thursday.
The explosion comes as the fossil fuel industry is reeling from a succession of deadly and costly accidents, including the BP spill in the Gulf of Mexico. It also comes two months before California is to vote on a proposition backed by the oil industry that would suspend a tough law intended to curb carbon emission and push the state to explore different forms of energy.
Whatever the cause of the explosion, the effects were devastating. On this sparkling late summer day, residents walked up and down the streets, many with sweeping views of the hill and bay, peering over police lines to get a glimpse of the damage. They traded stories of what happened when they heard the explosion; even in this part of the state, hardened by living in the heart of earthquake territory, residents seemed rattled by the randomness of the destruction.
“I got the dogs, kids and wedding pictures and started running,” said Rhonda Boone, who emerged from her house to see walls of flame higher than the trees.
Vijay Duggal, 60, was watching a football game. “I thought we’d been attacked by a missile,” he said. “Everything was shaking. I thought, This is the end.”