SAN FRANCISCO (Reuters) - Coroners on Sunday examined skeletal remains found in the charred rubble of homes leveled by a natural gas explosion, as authorities sought to account for four people missing three days after the blast.

The official death toll from the explosion and ensuing firestorm that engulfed dozens of homes in the San Francisco suburb of San Bruno on Thursday night stood at four on Sunday, said Robyn Thaw, a spokeswoman for San Mateo County.

The tally was expected to climb once medical examiners completed tests on skeletal remains recovered from the site, though Thaw said it was not immediately clear whether all the bone fragments were of human or animal origin.

Officials said they had revised the number of those unaccounted for back to four on Sunday afternoon after locating two others previously included among the missing.

If all or most of those still missing were to have perished, it would mark the biggest loss of life from a U.S. natural gas pipeline accident in a decade, and one of the nation’s deadliest such mishaps on record.

More than 50 people were injured in the accident, which occurred when an underground section of a gas transmission line 30 inches in diameter ruptured in an enormous fireball that destroyed or heavily damaged nearly 50 houses.

The damaged pipeline, installed in 1956, belongs to Pacific Gas & Electric Co, a unit of PG&E Corp

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A spokesman for the National Transportation Safety Board, which investigates pipeline accidents, said officials were preparing to ship a large section of pipe that was blown out of the ground, along with two smaller pieces from either end of the rupture, back to Washington for further inspection.

An initial examination found the ejected section of pipe consisted of several segments welded together, but it was not known whether this was a factor, NTSB officials said.

PG&E has said it was poring through its telephone logs to see if it could confirm media accounts of residents calling the utility days before the blast to report the smell of gas.

The NTSB has yet to identify anyone “who has claimed that they smelled the gas or made the phone call,” agency investigator Ravi Chhatre told Reuters.

According to PG&E, the gas line in question was inspected earlier this year. But NTSB spokesman Peter Knudson said the agency was seeking records to show when the section of pipe that ruptured was last examined.

The highest death toll in a U.S. natural gas pipeline accident is believed to have been from a 1965 blast in Natchitoches, Louisiana, that killed 17 people and prompted enactment of the 1968 Natural Gas Pipeline Safety Act.

A natural gas pipeline blast near Carlsbad, New Mexico, in August of 2000, killed 12 campers who were members of an extended family. Five people died in a similar explosion in Walnut Creek, California, in 2004.