San Bruno, Calif. - Federal investigators Friday combed the scene of one the state's deadliest natural gas explosions, trying to determine why a decades old, high-pressure gas pipeline exploded under a San Francisco Bay Area suburb.

The blast killed at least four residents, injured dozens of others and ignited fires that destroyed 37 homes in the hilly San Bruno neighborhood.

The National Transportation Safety Board, which is leading the investigation, and the pipeline owner, Pacific Gas & Electric, said they were looking into reports that residents smelled gas in the days prior to Thursday evening's explosion.

Among those missing and presumed dead were a 44-year-old employee of state agency that oversees the gas industry, and her teenage daughter, officials said. Seven people remained in critical condition Friday, including some with severe burns. There were still some badly charred homes to be searched, but fire officials said they had no reports of people still missing.

The blast left a gaping crater in the street above a 30-inch gas transmission line, the sort officials say are used to transport massive volumes of gas to residential and business distribution grids.

The wind-whipped inferno, reaching 1,000 feet high at one point, rapidly spread from house to house as smaller gas lines in the area burst open, officials said. Workers were unable to shut off the fuel supply for at least an hour.

When the smoke cleared Friday, investigators began picking their way past torched homes and burned out vehicles toward a huge piece of the massive steel pipeline, jutting out of the blackened ground. At an evening press conference, NTSB board member Christopher Hart said the force of the blast had thrown a large section of pipe out of the ground, an indication of the explosion's power. "It's an amazing scene of destruction," he said.

A final report on cause of the disaster will not be completed until late next year at the earliest, Hart said. Among the possibilities investigators will likely focus on is possible corrosion of the pipe, which has been a factor in pipeline failures, including an 2000 explosion of a 30-inch pipeline in New Mexico that killed a dozen people, experts said. PG&E said the San Bruno line was 40 to 50 years old.

On Friday, San Bruno police said they were treating the blast site as a possible crime scene until foul play is ruled out.

PG&E officials vowed to cooperate with the federal investigation, but did not say whether the company's pipeline caused the 6:30 p.m. PDT explosion.

"It does everyone a disservice to point fingers before any investigation of the facts has even begun," said PG&E spokesman Andrew Souvall.

Experts said explosions involving major gas transmission lines are rare because of redundant safety and testing requirements. "I was mystified," said Jim Moore, a University of Southern California engineering professor and infrastructure specialist. "If this was the result of a routine failure that was somehow unanticipated, then we need to begin inspecting these transmission systems very carefully."

The San Mateo County Coroner's Office has not officially identified the four who died in the explosion. But the president of the state's utility commission said two of those presumed dead are Jacqueline Greig, a commission employee, and her daughter.

"She lived right at the spot where it blew," Michael Peevey said of Greig. "She and a younger daughter were in the house. Her husband and the older daughter were at the daughter's school."