A 10-member team of NTSB investigators has traveled to the site of the explosion since Wednesday to examine the ruptured pipe. An examination on Thursday indicated that the pipe was about 70 percent thinner than it should have been to sustain pressure of 921 pounds per square inch measured at the time.
By Friday, Sumwalt said, investigators found evidence of external corrosion to the pipe that exploded. Investigators would examine what caused the corrosion and if a detection system was in place to find and control the erosion, he said.
Officials with NiSource have refused to answer questions about the shutoff equipment on the pipeline, or about their response to the explosion.
Sumwalt said the company provided records detailing what the pipe is made of and what detection systems are in place. However, investigators want to verify that information before making it public.
He said that, sometimes, gas companies provide inaccurate information, like Pacific Gas and Electric Co. officials did in 2010 when a natural gas transmission pipeline exploded and killed eight people in San Bruno, Calif.
"In San Bruno," he said, "what they told us was in the ground was not what actually was in the ground."
NTSB investigators said in that case, the 95 minutes PG&E took to stop the flow of gas and isolate the ruptured line "was excessively long and contributed to the extent and severity of property damage and [increased] the life-threatening risks to the residents and emergency responders."
Sumwalt said investigators would crate up a section of the ruptured pipe in Sissonville to send to the NTSB's lab in Washington, D.C., by nightfall tonight. They previously had hoped to extract the pipe on Friday but a physical onsite examination of it took longer than expected.
Investigators also have plans to extract long sections from two pipes nearby to see if the pipes are corroding or close to rupturing.
The NTSB team will remain in Sissonville until about the middle of next week before heading back to Washington for the analysis phase of the investigation, Sumwalt said.
The NTSB is an independent government agency that investigates civil transportation incidents. The board has no regulatory or enforcement authority, but instead issues reports that detail why incidents occur and recommends changes that industry and regulatory agencies could make to avoid future incidents.
The NTSB advocates that gas companies provide detailed information and locations of their pipelines to emergency responders and control room operators. The agency also recommends that gas companies install automatic shutoff valves in certain areas along pipelines.
Reports usually take about 12 months to complete, Sumwalt said, adding that if NTSB officials discover an emergency at any time, they would issue immediate recommendations.
Staff writer Ken Ward Jr. contributed to this report.
Reach Travis Crum at travis.c...@wvgazette.com or 304-348-5163.