Explosions could be caused by non-uniform drops in pressure throughout the pipe, creating a "jackhammer effect," he said.
So far, investigators have not determined what material the pipe is made of.
The pipe will be extracted Friday and sent to NTSB's lab in Washington, D.C., for a detailed material examination.
Investigators also will extract about 10 feet of nearby undamaged pipe to see if it also presented an explosion risk.
On Wednesday, Chevalier Mayes, communications manager for NiSource, did not respond to questions about the age of the pipeline or requests for details about any of the company's recent examinations of the pipe.
Addtionally, Mayes would not describe what sort of shutoff systems the company had in place at the pipeline.
The pipe that blew up runs east to west, perpendicular to I-77. Two other transmission pipelines are nearby, one about 30 inches in diameter and the other about 26 inches in diameter.
Sumwalt said Columbia Gas, the Kanawha County Sheriff's Department, the U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration and the West Virginia State Police are parties to the NTSB investigation. All will provide expertise when needed, he said.
Sumwalt said he would hold another briefing Friday at 5 p.m.
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.
Reach Travis Crum at travis.c...@wvgazette.com or 304-348-5163.