Federal investigators probing last week's massive gas explosion here found an unusually built section of pipe in the ruins, focusing attention on the pipeline operator's inspections.

The operator, PG&E Corp., was midway through a federally mandated program to inspect and make emergency repairs to its most important gas-transmission pipelines—especially those that pass through populated areas such as San Bruno—when the explosion happened, killing at least four residents.

Authorities said Sunday afternoon the death toll was still officially four, though they found additional skeletal remains. The San Mateo County coroner said his office needed to run tests to determine how many bodies there are and if they are human.

Four people remain missing, local officials said. Police said they were continuing to search the devastated area for more bodies. The flames left 58 houses destroyed or uninhabitable, officials said.

The blast, which erupted just after 6 p.m. on Sept. 9, threw a section of pipe 100 feet away—a section that National Transportation Safety Board investigators said might hold clues to what caused the blast that left a 167-by-26-foot crater.

The 28-foot length of pipe consisted of several smaller segments that were welded together in an unusual configuration. It also contained a long seam that ran the length of the pipe, the NTSB said.

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