Sept. 13, 2010 -- The natural gas pipe that leveled part of San Bruno, California was packed up and trucked out by the National Transportation Safety Board today as its most critical piece of evidence -- even as residents just down the street from where the explosion occurred finally returned home.
Last week's massive gas explosion in San Bruno destroyed dozens of homes -- killing at least 4 and leaving more than 60 people injured. The explosion is believed to have been caused by the gas pipe rupture underground.
The people who live on the street where this are still nervous, and there are still PG&E workers using electronic sniffers to check for more leaks under the streets.
The pipe which was installed in 1956. Investigators say the pipe has a long seam where it was welded together and may have been susceptible to corrosion.
Also, they've also found a hodgepodge of small pieces called "pups," each individually welded in place to help the pipe dip under a road. More modern pipes are simply bent to shape, which leaves fewer weld points that could fail or catch fire. Pipelines elsewhere have been ignited by sources as ordinary as a nearby stove or a spark from a car.
Investigators now say they have found no hard evidence that neighbors formally complained about the smell of gas leading up to the blast. They're now asking people to report whether they noticed any brown or dying grass or trees near the pipe that could have indicated a leak.
Meanwhile residents whose homes suffered only minor damage are just trying to clean up and get their lives back in order on a street that is still anything but normal.
"You go back into the house and you close the door, and everything is like normal until you come back out of the house, and then you see all the construction, you see all the homes completely gone." said San Bruno resident Michael Sah.
Pipelines Run Under Neighborhoods Across the Country
In a suburb of San Diego last night, more trouble seeped from underground. The smell of natural gas forced dozens of people from their homes. In Illinois, hundreds of workers scrambled to contain crude oil that gushed for three days from a ruptured main.
According to Carl Weimer of the Pipeline Safety Trust, "There is a pipeline accident every other day and every five days someone is injured or killed, so there's still way too many incidents. It's something we need to get a clear handle on before it gets worse."
San Bruno Explosion: Could One Happen Where You Live?
There are two and a half million miles of pipeline crisscrossing the nation. Much of that infrastructure is at least 40 years old and often in decay.
According to Weimer, a vast majority of transmission pipelines do not require inspection. Only pipelines near natural resources or pipelines near population centers are subject to mandatory inspection. Of those, only 7 percent of those major lines run anywhere near a neighborhood. However, while utility companies know where pipelines are buried, residents may not.
Residents of a Chicago neighborhood near a large natural gas pipeline claim they had no idea the pipeline existed, and there are no signs there to indicate a major pipeline there. Utilities are required by law to mark pipelines clearly and notify residents about the oil and gas lines running beneath them.
How Do I Determine if There's A Gas Leak In My Home or Neighborhood?
ABC News talked to Consolidated Edison, the utility company in the New York City area, about the top signs of a natural gas leak. Here are some things to look for to keep you and your family safe.
Smell: Natural gas is colorless and odorless but utility companies often add an odor to the gas so that leaks can be detected. At the start of heating season, some utility companies will send scratch-and-sniff samples so that you know what to look for. Typically, it's a rotten egg smell.
Hear: If you hear a roaring, hissing or whistling sound, it could be a leak.
See: Look for a white cloud or a misty fog. If you notice bubbles in standing water or vegetation that appears dead or seems to be dying for no reason, there might be a natural gas leak.
If you do suspect a leak, you should leave your house immediately. Open your windows if there is a faint odor. Don't do anything that could create a spark, meaning don't turn on and off your lights or appliances or use your telephone or cell phone. Ringing a doorbell or even starting your car could create a spark.
ABC News' Neal Karlinsky, Barbara Pinto contributed to this report.