SAN BRUNO, Calif. — 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.

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.

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.”

And once a high-pressure pipeline fails, he added, 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.

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.

PG&E President Chris Johns said the company has heard the reports of a gas odor in the area before the blast.

“Right now, we haven’t got confirmation about that, but we have records that we are going back right this minute to try to confirm what exactly those phone calls look like and when they occurred, and we will report back as soon as we know something,” he said.

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 access the site of the ruptured line Friday because it was covered with water.

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.

A National Transportation Safety Board’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.

PG&E has not returned calls seeking a response to its history.

On Friday 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 completely 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.

Four firefighters suffered minor smoke inhalation injuries and were treated and released, Haag said.

Haag said crews walked through the neighborhood Friday morning and revised the damage estimate to 38 structures destroyed and seven significantly damaged. Dozens of other homes suffered less severe damage in the fire, which burned 15 acres.

Haag said Friday afternoon a quarter of the homes were still too hot to search. He said he didn’t know of anyone confirmed missing, though officials were still waiting for all residents to check in.

A 44-year-old woman and her young daughter are among those feared dead, although officials have not released any names.

Agustin Macedo said his 44-year-old daughter, Jacqueline Greig, and his granddaughter were killed. Macedo said he was too upset to give any more information, including how he knew his family members’ fate. The granddaughter’s age and name weren’t known.

Greig lived in a house just yards from the source of the blast. Greig worked at the California Public Utilities Commission. Executive Director Paul Clanon announced to staff Friday morning that Greig was missing.

Shares of PG&E Corp. fell sharply Friday. PG&E shares closed down $4.03, or about 8 percent, at $44.21 during the regular session. The drop slashed $1.57 billion from the company’s market capitalization, based on its 390.75 million shares outstanding at July 29.

___

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.