Federal investigators Friday combed the scene of one of the state's deadliest natural gas explosions, trying to determine why a decades-old, high-pressure gas pipeline exploded under a Bay Area suburb.
The blast killed at least four residents, injured dozens of others and ignited fires that destroyed 37 homes in the hilly neighborhood of San Bruno.
The National Transportation Safety Board, which is leading the investigation, and the pipeline owner, Pacific Gas & Electric, said they were looking into reports that residents smelled gas in the days before Thursday evening's explosion.
Among the dead were a 44-year-old employee of a state agency that oversees the gas industry and her teenage daughter, officials said. Eight people remained hospitalized Friday, including some with severe burns. There were still some badly charred homes to be searched, but fire officials said they had no reports of people still missing.
The blast left a gaping crater in the street above a 30-inch gas transmission line, the sort officials say is used to transport massive volumes of gas to residential and business distribution grids.
The wind-whipped inferno, reaching 1,000 feet high at one point, rapidly spread from house to house as smaller gas lines in the area burst open, officials said. Workers were unable to shut off the fuel supply for at least an hour.
When the smoke cleared Friday, investigators began picking their way past torched homes and burned-out vehicles toward a huge piece of the massive steel pipeline, jutting out of the blackened ground. At an evening news conference, NTSB board member Christopher Hart said the force of the blast had thrown a large section of pipe out of the ground, an indication of the explosion's power. "It's an amazing scene of destruction," he said.
A final report on the cause of the disaster will not be completed until late next year at the earliest, Hart said. Among the possibilities investigators will probably focus on is possible corrosion of the pipe, which has been a factor in pipeline failures, including the 2000 explosion of a 30-inch New Mexico pipe that killed a dozen people, experts said. NTSB officials said the San Bruno line was installed in 1956.
On Friday, San Bruno police said they were treating the blast site as a possible crime scene until foul play is ruled out.
PG&E officials vowed to cooperate with the federal investigation, but did not say whether the company's pipeline caused the 6:30 p.m. explosion.
"It does everyone a disservice to point fingers before any investigation of the facts has even begun," said PG&E spokesman Andrew Souvall.
Experts said explosions involving major gas transmission lines are rare because of redundant safety and testing requirements. "I was mystified," said Jim Moore, a USC engineering professor and infrastructure specialist.
"If this was the result of a routine failure that was somehow unanticipated, then we need to begin inspecting these transmission systems very carefully."
The explosion occurred as PG&E and other pipeline operators were working to comply with a costly, federally mandated inspection and safety management program.
According to experts who monitor federal and state agencies for ratepayer groups, PG&E was required to inspect half of its transmission pipelines by the end of 2008 and the other half by the end of 2012. The utility also was expected to carry out a regular re-inspection program.
"We don't know what the status of the project is," said William B. Marcus, a utility economist who works for the Utility Reform Network on PG&E rate matters before the California Public Utility Commission.
"But we do know that PG&E has been spending" millions annually to get the inspection program completed.
PG&E has had extensive problems with a program to inspect leaks in its extensive distribution network stretching from the California-Oregon border to Bakersfield in the southern San Joaquin Valley, according to a company report and industry watchdogs.
Distribution pipes, which deliver natural gas to neighborhoods and individual homes, are smaller in diameter and operate under less pressure than giant transmission pipes such as the one that exploded in San Bruno.
"PG&E's gas leak detection program failed miserably earlier this decade," Marcus, whose group advocates for ratepayers, wrote in a report to the California Public Utilities Commission in June.
Marcus said he was able to gain some insight into the program's difficulties because the PUC released a PG&E PowerPoint presentation on the program.
The presentation noted that a 2007 re-survey of leaks in the North Coast Division "found deficiencies" including "record falsification" in earlier surveys performed in 2004-07.