Investigators have said alarms didn't sound at Columbia Gas Transmission's Charleston control room during the explosion.
A Columbia Gas control room worker first learned about the accident from another company, Sumwalt said. The worker received a call 10 minutes after the blast from a controller at Cabot Gas, who had been contacted by someone on the outside.
The NTSB plans to interview the Cabot Gas employee and attempt to locate the outside caller, Sumwalt said.
Earlier Friday, Columbia Gas Transmission chief executive Jimmy Staton released an open letter to the community saying the company is committed to helping investigators pinpoint the cause of the explosion and ensure the pipeline is safe.