Investigators said they believe the pipe, which was first installed in 1956, may have been susceptible to corrosion.
They found a hodgepodge of small pieces called "Pups," each individually welded in place to help the pipe make a dip under the road. Modern pipes are simply bent to shape to leave fewer weld points prone to failure.
The state has ordered Pacific Gas and Electric to survey all of its natural gas lines in California following the deadly San Bruno explosion.
San Bruno residents are returning to a neighborhood in ruins. Some homes are still standing but there's a lot of damage. Officials have collected metal fragments of the pipeline as they start trying to figure out what caused the explosion.
Nearly 50 homes were destroyed in Thursday night's blast and at least four deaths have been confirmed. Investigators are still trying to confirm exactly how many people died.
Dozens of other homes were scorched and, in some cases, severely damaged.
Meanwhile, local and federal officials are probing the cause of the explosion that blew a segment of pipe 28 feet long onto the street some 100 feet away, creating a crater 167 feet long and 26 feet wide.
Documents submitted to regulators three years ago by the utility, Pacific Gas and Electric show a nearby section of the pipeline was within the "top 100 highest risk line sections" in the utility's territory and had an unacceptably high risk of failure.
It was due to be replaced because it ran through a heavily urbanized area. The company had also called the section that ruptured a "high consequence area."
Federal officials said that classification requires more stringent inspections.
Investigators said they want to hear from any San Bruno residents who smelled gas in the days before the deadly blast. So far, investigators with the National Transportation Safety Board said they haven't seen any record of gas leak complaints before the explosion.
Residents who fled their homes started returning on Sunday. Homes have been tagged to indicate the amount of damage. The only people being allowed to stay are those with green tagged homes.
"We feel relief. We feel guilt," said San Bruno residents Pat and Fred Gillan. "Because ours is still there and most of them are gone."
"My heart goes out to my neighbors and the lives lost. I'm really shocked at what I'm seeing right now," said another San Bruno resident Walter McCaffrey.
Many have expressed anger over the situation.
"PG& E and the insurance companies really need to step up and step up quickly and get us into long term homes," said Bill Magoolaghan, another victim of the explosion. "I think they need to buy us new homes."
PG&E said the pipeline in the area was last checked for leaks in March and no problems were found.
If you would like to help the residents affected by the San Bruno fire, monetary donations can be sent to:
American Red Cross Bay Area Chapter
85 Second Street, 8th Floor
San Francisco, CA 94105
You can also call (888) 4-HELP-BAY or visit the American Red Cross Bay Area Chapter's website to donate online, http://www.redcrossbayarea.org.
The Associated Press contributed to this report.