The death toll wasn't the worst in pipeline history. An incident 10 years ago in Carlsbad, New Mexico, killed 12. Pipeline blasts in the past five years have killed 60 and injured 230.
Though roughly half these incidents were the fault of parties other than utilities (builders or cable companies that accidentally dug into underground pipes), pipeline operators dug into their own pipes in at least two dozen cases. Other incidents for which they were responsible involved corrosion, faulty equipment and operator error.
The San Bruno incident was caused by a pipe that ruptured because of regular changes in gas pressure, according to federal investigators.
The age of a pipeline matters less than inspection and maintenance, said Carl Weier, head of the Pipeline Safety Trust, a government-financed watchdog group. "Most of the pipelines in this country are 40 to 50 years old. If properly maintained, they don't present a danger."
But even a new pipeline, he said, will fail if not well-inspected and maintained. Corrosion caused the Carlsbad event, according to inspectors who examined the wreckage. Weier said the danger of future explosions could be defused by better and more frequent inspection, especially in rural areas, where pipelines get a thorough going-over only once every seven years.
The Associated Press contributed to this story.