A local fire chief said a gas line explosion likely caused the blast that sent flames tearing through the streets, incinerating possibly more than a dozen homes just after 6 p.m.
"We believe it's a high-pressure gas line that's blown," San Bruno Fire Chief Dennis Haag told KPIX-TV.
Pacific Gas and Electric, the utility company that serves the San Francisco Bay area, had crews in the area investigating the possibility of a natural gas explosion but had no additional information, said spokesman J.D. Guidi.
Following the initial blast, flames reached as high as 60 feet in the air as the fire fueled itself on burning homes. Planes and helicopters flew over the neighborhood dumping water in an effort to stanch the flames.
Witnesses said the blaze was preceded by a loud explosion and huge fireball. They described seeing residents fleeing for safety and rushing to get belongings out of their burning homes.
Connie Bushman returned home to find her block was on fire. She said she ran into her house looking for her 80-year-old father but could not find him. A firefighter told her he had left, but she had not been able to track him down.
"I don't know where my father is, I don't know where my husband is, I don't know where to go," Bushman said.
Victims suffering from serious burns began arriving at San Francisco Bay area hospitals shortly after the blast. An estimate of the number of injured was not immediately available. Hospitals reported receiving at least four victims in critical condition but anticipated more.
Jane Porcelli, 62, said she lives on a hill above where the fire is centered. She said she thought she heard a plane overhead with a struggling engine.
"And then you heard this bang. And everything shook except the floor, so we knew it wasn't an earthquake," Porcelli said. "I feel helpless that I can't do anything. I just gotta sit by and watch."
Stephanie Mullen, Associated Press news editor for photos based in San Francisco, was attending children's soccer practice with her two children and husband at Crestmoor High School when she saw the blast at 6:14 p.m.
"First, it was a low deep roar and everybody looked up, and we all knew something big was happening," she said. "Then there was a huge explosion with a ball of fire that went up behind the high school several thousand feet into the sky.
"Everybody grabbed their children and ran and put their children in their cars," Mullen said. "It was very clear something awful had happened."
Several minutes later, Mullen was near the fire scene, about a half-mile away in a middle-class neighborhood of 1960s-era homes in hills overlooking San Francisco, the bay and the airport. She said she could feel the heat of the fire on her face although she was three or four blocks away from the blaze. It appeared the fireball was big enough to have engulfed at least several homes.
"I could see families in the backyards of the homes next to where the fire was, bundling their children and trying to get them out of the backyards," she recounted.
She said people in the neighborhood were yelling, "This is awful," "I live down there," and "My family is down there." Judy and Frank Serrsseque were walking down a hill away from the flames with a makeshift wagon carrying important documents, medication and three cats.
Judy Serrsseque said she heard an explosion, saw that fire was headed toward their home and knew they had to leave. As they fled, they said they saw people burned and people struggling to get their things out of burning houses.
"We got everything together, and we just got out," Judy Serrsseque. "Mostly we're wondering if we have a house to go back to."