SAN BRUNO — All that was left of some houses Friday were chimneys, rising from still smoldering ruins. Burned-out cars sat along ash-covered streets. And a rescue worker with a dog searched door to door for missing people.
The day after a gas line ruptured and a towering fireball roared through a suburban San Francisco neighborhood, killing four people, officials were trying to determine what led to a blast that raised questions about the safety of similar lines that crisscross towns across America.
"It was pretty devastating," Fire Chief Dennis Haag said. "It looks like a moonscape in some areas."
At least 50 people were hurt, with seven suffering critical injuries in the explosion Thursday evening that left a giant crater and laid waste to dozens of 1960s-era homes in the hills overlooking San Francisco Bay.
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The utility that operates the 30-inch diameter line said it was trying to find out what caused the steel gas pipe to rupture and ignite. Federal pipeline safety inspectors were also on the scene.
Some residents said they smelled gas in the neighborhood over the past several weeks. The utility said it was checking its records for the complaints, but added that none of its crews were at work on the line Thursday.
Compared to the tens of thousands of miles of gas pipelines across the country, accidents are relatively rare. In 2009, there were 163 significant accidents involving natural gas pipelines, killing 10 people and injuring 59.
Experts say the nation's 296,000 miles of onshore natural-gas lines routinely suffer breakdowns and failures.
More than 60 percent of the lines are 40 years old or older and almost half were installed in the 1950s and 1960s, according to a recent analysis by the Pipeline Safety Trust, a nonprofit advocacy group based in Bellingham, Wash.
Most of the older pipelines lack anticorrosion coatings that are prevalent in the industry today, said Carl Wei- mer, executive director of the trust.
Federal investigators will analyze the pipeline's condition, along with its maintenance history, pressure levels and the safeguards put in place to prevent pressure from building up, National Transportation Safety Board vice chairman Christopher Hart said. The NTSB will also look at the training and experience of the people who operated the pipeline and screen them for alcohol and drugs.
PG&E President Chris Johns said the company has heard the reports of a gas odor in the area before the blast.
By midafternoon Friday, the utility could not confirm the residents' reports of gas odors, but said it was "looking into it."
The damaged section of pipe was isolated and gas flow to the area was stopped. Haag said PG&E crews were still not able to examine the site of the ruptured line Friday because it was covered with water.
Evacuees will have to wait until at least today to return to the fire zone to see their homes or what was left of them, officials said.
Haag said crews walked through the neighborhood Friday morning and revised the damage estimate to 37 structures destroyed and eight significantly damaged. Dozens of other homes suffered less severe damage in the 15-acre fire.
Search and rescue teams and cadaver dogs completed a search of the area except for a few remaining hot spots and turned up no more victims, leaving the number of confirmed fatalities at four, said Matt Bettenhausen, secretary of the California Emergency Management Agency.