Local utility officials said they believe the source of the blast was a large natural gas pipe that ran underground through San Bruno, where some houses in several blocks near San Francisco International Airport were reduced to smoldering ruins and authorities conducted emergency evacuations.
The San Mateo County coroner's office confirmed one death related to the explosion.
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More than 30 people were treated at a local Kaiser Permanente hospital in San Bruno, while others were rushed to burn units at other medical facilities, according to hospital spokesman Karl Sonkin.
"This is really a tragedy," Sonkin said. "We are caring for people as best we can."
A number of fires were still raging out of control nearly four hours after the initial explosion, and parts of the affected neighborhoods looking like a war zone with several homes up in flames, debris littering the streets.
Several fire crews were on the scene attempting to douse what witnesses described as a massive "ball of fire" that billowed skyward after a huge explosion.
"I heard a sound like a low flying plane, then all of a sudden the house shook," said Tina DiIoia, who was with her baby in their condominium in the town of San Bruno, just south of San Francisco, when the explosion occurred about a half-mile away.
"Then there was another explosion. I went outside and there was debris falling from the air."
Flames had spread to more than 20 homes and people were being evacuated from areas downwind of the inferno, while helicopters and airplanes were seen dumping water and fire retardant chemicals in an effort to stanch the flames.
"It looks like the entire mountainside is burning," DiIoia said as she and her husband took their baby and evacuated the town.
Local media reported more than 45 homes had caught fire, and hundreds of dazed residents had gathered at an evacuation site at a nearby shopping center or at shelters manned by the Red Cross in and around San Bruno.
The blast so close to the busy airport had initially sparked fears of terrorism, ahead of Saturday's ninth anniversary of the September 11, 2001 attacks, but there was no information suggesting a link.
