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Federal officials say working groups are being formed in an effort to find out why a gas line exploded in the San Francisco suburb of San Bruno.
National Transportation Safety Board vice chairman Christopher Hart called the neighborhood "an amazing scene of destruction."
The blast Thursday night set off a fire that swept through a neighborhood destroying at least 38 homes, while dozens more were seriously damaged, officials said.
California Lieutenant Gov. Abel Maldonado reported four dead, but a state senator and local fire officials tell Fox News that at least six people perished in the blaze. At least 50 people were hurt, 7 of them critically.
Hart says federal investigators will analyze the 30-inch diameter pipeline's condition, along with its maintenance history, pressure levels and the safeguards put in place to prevent pressure from building up.
The National Transportation Safety Board, which sent a four-member team to San Bruno, will also look at the training and experience of the people who operated the pipeline and screen them for alcohol and drugs.
The NTSB's duties include investigating pipeline accidents.
Maldonado, acting governor while Gov. Arnold Schwarzenegger is in Asia on a trade mission, said a natural gas line ruptured in the vicinity of the blast at around 6:24 p.m. on Thursday. He said authorities are still working to confirm what caused the rupture.
"Our hearts go out to those impacted by this horrible disaster," he said. "Without warning many of these people's lives have been changed forever."
The gas explosion shot a fireball more than 1,000 feet in the air and sent frightened residents fleeing for safety and rushing to get belongings out of burning homes.
San Bruno Mayor Jim Ruane said the death toll and number of those injured were expected to rise.
"The sun is shining over there but there is still a dark cloud over this city," Ruane told reporters. "You know the numbers...unfortunately the numbers are going to get higher."
Maldonado declared a state of emergency in San Mateo County, freeing up funds to help fight the blaze. The Federal Emergency Management Agency approved a fire management assistance grant to the city.
The blast left a crater the size of an intersection and sent flames tearing across several suburban blocks in San Bruno.
Resident 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.
After the initial blast, flames reached as high as 100 feet as the fire fueled itself on burning homes, leaving some in total ruins and reducing parked automobiles to burned out hulks. At least 120 homes also suffered serious damage.
As of Friday afternoon, the fire was completely contained with some "hot spots," Maldonado said. He said that the search for victims is 75 percent complete.
Maldonado also said that he's signed an executive order that directs state resources to the victims, including unemployment assistance.
Pacific Gas and Electric Co. said in an e-mailed statement Thursday that "If it is ultimately determined that we were responsible for the cause of the incident, we will take accountability."
The company said a damaged section of the steel gas pipeline had been isolated and gas flow had been stopped. About 300 customers were without gas service and about 700 without electricity at 4 a.m. Friday.
More than 100 people were being sheltered at nearby evacuation centers, but no estimate of the number of residents missing was available.
San Bruno Fire Capt. Charlie Barringer said the neighborhood was engulfed by the time firefighters arrived on Thursday, even though the fire station was only a few blocks away. He said the blast took out the entire water system, forcing firefighters to pump water from more than two miles away.
Victims suffering from serious burns began arriving at San Francisco Bay area hospitals shortly after the blast. As of late Thursday, hospitals reported receiving about 20 injured patients -- several of whom were in critical condition -- and they anticipated getting more.
Jane Porcelli, 62, said she lives on a hill above where the fire was 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" 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."
Fox News' Adam Housley and the Associated Press contributed to this report