SAN BRUNO, Calif. (AP) -- Federal authorities are probing a natural gas pipeline and how it was maintained as they investigate the thunderous line explosion and raging inferno that devastated this suburban San Francisco neighborhood. City leaders called for a town hall meeting Saturday to start San Bruno's healing process.
Officials were trying to determine what led up to the blast that killed at least four people, injured dozens of others and raised questions about the safety of similar lines that crisscross towns across America.
"It looks like a moonscape in some areas," Fire Chief Dennis Haag said Friday.
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. Authorities had no reports of anyone still missing or unaccounted for.
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 on the scene Friday afternoon.
"It was just an amazing scene of destruction," National Transportation Safety Board vice chairman Christopher Hart said.
He said 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. 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.
At an evacuation center, residents anxiously awaited word on the fate of their homes.
Others, like Freddy Tobar and his wife Nora, thought about the house they lost. He saw flames shooting up outside his window and then through his home. He grabbed his Chihuahua and ran outside, getting second degree burns on his arms and the side of his face.
The couple saw the house burning to the ground on the news, and returned Friday to find it destroyed.
"We have to start from zero again. When you start remembering it gets too sad," Nora Tobar said.
"But the most important thing is that we're alive," she said.
Jacquelin Greig, 44, her daughter Janessa, 13, and Jessica Morales, 20, were identified by the San Mateo County Coroner's Office as having died in the fire. The fourth person killed hasn't been identified.
Greig lived in a house just yards from the source of the blast. In her job with the California Public Utilities Commission, she worked to protect consumers from soaring monthly gas bills or dangerous pipeline expansions, co-workers said.
"This is so difficult for us because we're such a small group," said her co-worker Pearlie Sabino. "She does a lot of cases related to natural gas, that's the irony of it."
State Assemblyman Jerry Hill, who represents San Bruno and surrounding cities, said he has heard multiple reports from constituents who had alerted PG&E of gas odors in the neighborhood before the disaster.
The residents "deserve to know if PG&E used the correct procedures in the days and weeks leading up to this disaster," Hill said. 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.
Transmission lines like the one that burst in San Bruno deliver natural gas from its source to distribution lines, which then carry it into neighborhoods before branching off into homes.
Over the past two decades, federal officials tallied 2,840 significant gas pipeline accidents nationwide -- including 992 in which someone was killed or required hospitalization, according to the Pipeline and Hazardous Materials Safety Administration.
Those accidents killed 323 people and injured 1,372.
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 Weimer, executive director of the trust, which was set up following a 1999 explosion that killed three people in Bellingham.
"The industry always says that if you take care of pipelines, they'll last forever," Weimer said. "But what we see over and over again is companies are not doing that and corrosion and other factors are causing failures."
Once a high-pressure pipeline fails, he said, anything can trigger a deadly blast. A cigarette or rocks smashing as high-pressure gas shoots by. Even someone answering a cell phone can cause a spark, because it is battery-powered, Weimer said.
This is not the first time a deadly explosion occurred on a PG&E gas line. The utility has had 19 significant pipeline incidents since 2002, but there was only one fatality, according to records provided by the trust.
In 2008, the state regulators inspected a leaky PG&E pipeline in the Sacramento suburb of Rancho Cordova that had been repaired, and found that the company wasn't properly training its workers to recognize potentially dangerous leaks.
PG&E agreed to update its safety training, and a deadline was set for Dec. 31, 2008.
On Christmas Eve, the pipeline exploded, killing a 72-year-old man and injuring five others.
NTSB's final report on the blast concluded that PG&E used a wrong pipe to repair the gas line two years before and that residents had reported a gas smell before the explosion.
In response to the findings, the company said it had taken "extraordinary measures" to ensure a blast like that wouldn't happen again.
Contributing to this report were Associated Press writers Barbassa, Sudhin Thanawala, Terence Chea and video journalist Haven Daley in San Bruno; Dearen, Marcus Wohlsen and John S. Marshall in San Francisco; Garance Burke in Fresno, Calif.; Matthew Brown in Billings, Mont.; and Joan Lowy in Washington.