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.