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A massive explosion and fire on Sept. 9, 2010 destroyed 38 homes and left a large crater in a residential neighborhood of San Bruno

An investigation is under way into what caused the gas transmission line to rupture, resulting in the fiery destruction of a San Bruno neighborhood. 

No conclusions have been reached yet, but industry experts say the line could have been hit when someone was excavating or it could have deteriorated from corrosion.

The pipe that ruptured was not a distribution line that fed gas to homes in the neighborhood but rather a high-capacity transmission line bringing gas from the source to the Bay Area.

“The leading cause of pipeline accidents used to be internal corrosion, but really today the leading cause is excavation,” said Brigham McCown, a lawyer who worked in the U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration for four years.

Once the cause is determined, McCown said that hefty fines could be levied if PG&E is at fault.

“If this is caused by operator error, there will be substantial penalties,” McCown said.

Some residents of the neighborhood now say that the utility had a crew doing work there for several days before the devastating blast.

Shane Musunu and his mother, Gayle Musunu, live at the corner of Glenview and Claremont streets where the explosion occurred. Both say that PG&E workers were at that site for three or four days earlier this week. Residents complained to the work crew about smelling gas, the Musunus said, and they were told to close their garage doors to avoid smelling the gas odor.

PG&E has confirmed that one of its high-pressure natural gas transmission lines ruptured, causing the blaze that destroyed 38 homes, damaged 120 and killed at least three people. The explosion happened shortly aft 6 p.m. on Thursday and left a 15-foot-deep crater in the pavement.

PG&E officials said they had not determined what caused the company’s gas line to rupture. Visiting the scene, PG&E President Christopher P. Johns said the company would “do the right thing” if it was found to be responsible.

The California Public Utilities Commission, which regulates PG&E, said it was launching a formal investigation into the explosion. 

PG&E is required by law to maintain and monitor the gas lines, said Doug Burkhart, president of Smith & Dennison Construction in Livermore, which builds gas lines and developed technology to detect gas leaks.

"It's a highly regulated industry that requires a lot of testing monthly and annually," said Burkhart. "You don't just turn this thing on and walk away."

PG&E is responsible for monitoring the lines for leaks and corrosion, maintaining the shut off valves in good working order and having a disaster response plan. 

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