Investigators searched the smoldering rubble of a quiet, middle-class neighborhood tucked into the hills overlooking San Francisco on Friday, looking for answers to why a gas line ruptured and fed a giant fireball that killed at least four people.

Firefighters finally contained the fire Friday afternoon, but at least a quarter of the 15 acres of charred homes were still too hot to search. Crews with dogs went house to house and officials said there could be more casualties from the Thursday evening blast. Homes were left with just chimneys standing and smoke still rose from the burned-out wreckage.

"It was pretty devastating," said San Bruno Fire Chief Dennis Haag. "It looks like a moonscape in some areas."

Eight investigators were at the scene Friday and police blocked people from approaching the burned area.

"Until we can determine what caused that, we are protecting that like a crime scene, looking for evidentiary value items," said San Bruno Police Chief Neil Telford.

One man was arrested Thursday night on suspicion of looting and assaulting an officer, Telford said.

At least 50 people were hurt in the blast and fire, with eight in critical condition at area hospitals. The explosion left a giant crater and sent flames tearing through the neighborhood of 1960s-era homes in hills overlooking San Francisco, the bay and the airport.

Haag said he didn't know of anyone confirmed missing, though officials were still waiting for all residents to check in. Nearly 40 homes were destroyed and seven significantly damaged. Dozens of other homes suffered less severe damage in the fire.

Christina Veraflor, 41, of Napa, grew up in the neighborhood and said Friday morning that her 67-year-old mother's house was destroyed. Her mother, who had lived in the home for 40 years, was at the movies when the neighborhood erupted in flames.

"I woke up this morning and said, 'I'll go to my mom's and get this and get that.' But there is no mom's anymore," Veraflor said.

Pacific Gas and Electric Co. President Chris Johns said Friday morning a steel gas pipe ruptured about three feet underground just before 6:30 p.m Thursday, but crews still haven't been able to determine the cause of the rupture or the ensuing blast because they can't get close enough.

Haag said Friday afternoon they're still not able to access the site of the ruptured gas line because it's covered with water.

The damaged section was isolated and gas flow to the area has been stopped.

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.

"It was a continuous whooshing sound as if it was a fed fire," resident Michael Yost said. "It sounded like, you know, you would if you had a blow torch. It's that sound but, you know, a hundred times louder."

Veraflor said she smelled gas at the house during a visit six weeks ago but did not report the smell to the utility.

"You'd get a whiff of it, and it would dissipate," she said.

Johns said the company has heard the reports that some residents smelled gas in the area before the blast.

"Right now, we haven't got confirmation about that, but we have records that we are going back right this minute to try to confirm what exactly those phone calls look like and when they occurred, and we will report back as soon as we know something."

The mood was apprehensive at the evacuation center Friday, where several dozens of people came to get information and services.

Carlene Vasquez did not know what happened to her house and was anxiously waiting for news when her son came up and showed her a picture of her house, still standing, on his mobile phone.

"Oh my house," she said, crying. "That's my house."

Four firefighters suffered minor smoke inhalation injuries and were treated and quickly released, Haag said.

It's not the first time a deadly explosion on a PG&E gas line has devastated a Northern California neighborhood.

On Christmas Eve 2008 an explosion killed a 72-year-old man in the Sacramento suburb of Rancho Cordova, destroyed one home and seriously damaged others.

The National Transportation Safety Board's final report said PG&E used a wrong pipe to repair the gas line two years before the explosion. Rancho Cordova residents had reported of a gas smell in the area before the blast.

In response to the NTSB's findings, the company said it had taken "extraordinary measures" to ensure a blast like that would never happen again.

The NTSB has sent a four-member team to San Bruno to investigate Thursday's blast.

The pipeline involved in the accident is under the jurisdiction of the California Public Utility Commission. Transportation Secretary Ray LaHood said an official from his department would be on the scene "to make sure things are being done correctly."

"We are going to take 24/7 oversight to make sure everything is done correctly, LaHood told reporters in Washington.

However, LaHood added: "The responsibility for this falls under the state of California."

The National Transportation Safety Board, an independent federal agency, investigates pipeline accidents. The Transportation Department, which includes the Pipeline and Hazardous Materials Safety Administration, regulates pipeline safety.

Lt. Gov. Abel Maldonado, acting governor while Gov. Arnold Schwarzenegger was in Asia on a trade mission, declared a state of emergency in San Mateo County. He said PG&E had committed to providing temporary housing, food, clothing and other essential needs of the victims.

The Federal Emergency Management Administration made disaster relief funds available to cover 75 percent of the cost of fighting the San Bruno blaze. The money covers equipment, supplies and emergency work such as evacuations, shelter and traffic control.

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.

She rushed to the fire scene about a half-mile away and said she could feel the heat of the fire on her face although she was three or four blocks away from the blaze.

"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 said.

She said people in the neighborhood were yelling, "This is awful" and "My family is down there."


Contributing to this report were Associated Press writers Sudhin Thanawala in San Bruno; Marcus Wohlsen, Paul Elias, Trevor Hunnicutt and Jason Dearen in San Francisco; and Joan Lowy in Washington.