San Francisco, California (CNN) -- Thursday night's devastating gas pipeline explosion in San Bruno, California, raises a couple of critical questions: exactly how extensive and how safe are the nation's gas lines?
America's natural gas pipeline network consists of roughly 217,000 miles of interstate transmission and 89,000 miles of intrastate transmission, according to a February report from the Congressional Research Service. It also contains approximately 20,000 miles of field and gathering pipelines connecting gas extraction wells to processing plants.
On the whole, according to the report, "releases from pipelines cause few annual fatalities compared to other product transportation modes."
Companies operating natural gas transmission pipelines reported an average of one death per year from 2004 to 2008,the report said.
Video: 'I saw debris everywhere'
Video: Explosion a 'massive tragedy'
The Department of Transportation, it noted, reported 63 natural gas transmission pipeline accidents in 2008.
But San Francisco-based Pacific Gas and Electric, the company whose ruptured pipeline led to the San Bruno explosion, was also culpable in at least one other recent deadly incident, documents show.
An explosion and ensuing fire caused by a natural gas leak destroyed a house in December 2008 in Rancho Cordova, California, near Sacramento, according to the National Transportation and Safety Board.
One person died and five other people, including one utility worker and a firefighter, were injured.
PG&E had been notified of the gas leaks at the house where the blast occurred, according to the safety board's accident brief. Crews arrived to address the situation and a foreman had just finished speaking with a resident at the house and was about to investigate further when the explosion happened.
Sunny Dickson, identified by the Sacramento Bee as the granddaughter of the man who died, told the Sacramento Metropolitan Fire District that the family had returned home around noon on Christmas Eve, according to the safety board's brief.
The PG&E technician had knocked on their door earlier but did not see when the family returned.
Dickson said she did not see any PG&E vehicles nearby nor any warning notice on the door telling the family not to enter until leak investigators checked for gas.
Dickson went into the bathroom and soon after she heard a "whoosh" and two explosions, the safety board brief said. She escaped to the street with severe burns, as did her mother. Her grandfather, Wilbert Paana, died.
PG&E took responsibility for the blast. Almost a year later, a PG&E spokesman revealed that the company accidentally installed the wrong kind of pipe in 2006 when it fixed a gas leak at Paana's home.
National Transportation and Safety Board documents also showed that in May 2008, the California Public Utilities Commission, which regulates utilities, audited PG&E and cited the company for several violations, among them failing to provide "prompt and effective response to a notice of each type of emergency including ... gas detected inside or near a building."
Some of those violations had not been addressed when the Rancho Cordova blast occurred.
The safety board's investigation found that the first PG&E technician arrived on the scene almost an hour after the initial call but without proper equipment. An investigator with better equipment did not show up until just before the blast, almost three hours later.
In Thursday's San Bruno blast, a PG&E transmission line ruptured, leading to the massive blaze that killed at least three people and destroyed or damaged 169 homes. As of Friday afternoon, the accident was still under investigation, according to company spokesman Blair Jones.
Carl Weimer, head of the watchdog group Pipeline Safety Trust, told CNN Thursday that, unfortunately, when it comes to pipelines, for most of the public it's "out of sight, out of mind."
"I'm betting in San Bruno most people didn't even know the pipeline was under their neighborhood," he said.
Weimer noted that there can be multiple causes for a pipeline failure, including unintentional excavation-related damage and, more often, age and corrosion.
The proximity of multiple gas pipelines can also be a problem, he said.
The Pipeline Safety Trust is based in Bellingham, Washington -- site of a 1999 gas pipeline explosion that killed two children and an 18-year-old man while causing $45 million in damage, according to the Congressional Research Service report.
PG&E, which incorporated in California in 1905, is one of the largest combination natural gas and electric utilities in the United States, according to the company's website.
The company provides natural gas and electric service to about 15 million people throughout a 70,000-square-mile service area in northern and central California.
CNN's Moni Basu, Scott Bronstein, Dugald McConnell, Brian Rockus, Alan Silverleib, and Brian Todd contributed to this report
