The National Transportation Safety Board will investigate a natural gas explosion in San Francisco that sent flames into the sky for hours and damaged five buildings, an official said Thursday.
An eight-person team arrived in San Francisco on Thursday night to begin the lengthy process of gathering evidence and trying to determine what happened, spokesman Eric Weiss said.
The federal agency often investigates blasts on pipelines because they transport oil and natural gas, which it oversees.
"We're going to look at not only what happened but why it happened and see if there are any lessons to be learned for future safety," Weiss said.
California's workplace safety agency, Cal/OSHA, also is investigating.
A private construction crew digging on a street to install fiber-optic wires cut a natural gas line, igniting the fire Wednesday, Fire Chief Joanne Hayes-White said.
However, Weiss cautioned that it could be months or even years before the NTSB issues an official determination of the probable cause.
No one was injured but flames shot above the rooftops of three-story buildings and burned for more than two hours until Pacific Gas & Electric workers were able to shut off the gas fueling the fire. Some questioned why it took the utility so long.
State law required crews to dig by hand around numerous other pipelines before they were eventually able to "squeeze" a 4-inch (10-centimeter) plastic line, PG&E spokeswoman Melissa Subbotin said.
Because the fire was contained to a limited area, the utility said it had to weigh the threat from the flames with the risk of more drastic action on a cold day in San Francisco. Officials opted not to quickly shut off a massive transmission line as they would do in an earthquake, Subbotin said.
"Had we turned the gas off to a transmission system, we would have shut off gas to nearly the entire city of San Francisco," Subbotin said. "The objective of this was to turn the gas off safely and as quickly as possible."
The company stressed that the workers who cut the gas line are not affiliated with the nation's largest utility.
About 300 customers were affected by the gas line rupture. PG&E restored gas to the line Thursday and went door-to-door turning back on customers' pilot lights.
The company allegedly responsible for the massive, fiery gas rupture in San Francisco on Wednesday afternoon was identified as a third-party contractor for Verizon, but it was a subcontractor of that company who actually hit the line, state workplace inspectors said.
Verizon told KTVU-TV on Thursday that the contractor was Florida-based MasTec.
"We want to express our deep concern for those affected by the gas line rupture," MasTec executive John Higgins told the station in an email, adding that the company has started an investigation into the accident.
Frank Polizzi, a spokesman for Cal/OSHA, said an employee for a subcontractor, San Francisco-based Kilford Engineering, was operating an excavator when the gas line ruptured.
Kilford has no previous workplace violations, he told the station.
KTVU-TV said efforts to reach the company were unsuccessful.
Five buildings were damaged, including one housing Hong Kong Lounge II, a reservations-only dim sum restaurant that is a fixture on the city's "best of" lists. The fire began on the street in front of the restaurant.
Officials evacuated several nearby buildings, including a medical clinic and apartments, Hayes-White said. Vehicles on one of the city's main arteries were rerouted as authorities cordoned off the bustling neighborhood.
Caroline Gasparini, 24, who lives kitty-corner from where the fire ignited, said she and her housemate were in their living room when the windows started rattling. She looked up to see flames reflected in the glass.
"We went into crisis mode," Gasparini said. "We grabbed our shoes, grabbed our laptops and grabbed our passports and just left."
Gasparini said they saw employees of the burning restaurant run out the back door and people fleeing down the block.
Firefighters worked to keep the fire from spreading while PG&E crews worked to shut off the gas line.
The utility faces heightened scrutiny over its natural gas pipelines after one exploded in a neighborhood south of San Francisco in 2010, killing eight people and wiping out a neighborhood in suburban San Bruno.
A U.S. judge fined PG&E $3 million for a conviction on six felony charges of failing to properly maintain the pipeline, and the utility remains under a federal judge's watch in that case.