National Transportation Safety Board investigators had said previously that the company received no alarm to indicate a pressure drop at a 20-inch-diameter transmission pipeline that ruptured and exploded at about 12:41 p.m. Tuesday near Columbia Gas Transmission's Lanham Compressor Station at Rocky Fork.

Columbia Gas is a subsidiary of NiSource Gas Transmission and Storage.

It took workers more than an hour to find and isolate the section of transmission pipeline where the blast occurred and begin to shut off fuel to the line, investigators said this week.

Investigators would determine if the company's response in shutting off the gas was adequate, NTSB board member Robert Sumwalt told reporters Friday evening at the Wingate Hotel in South Charleston.

Investigators interviewed a Columbia Gas control room operator at the company's Charleston headquarters Thursday night, Sumwalt said. The operator told investigators that he learned of the explosion at about 12:50 p.m., approximately 10 minutes after it happened, when a Cabot Oil and Gas control room operator called him.

Cabot is an independent oil and gas operation.

"A Cabot Gas company controller had received a call from an outside caller ... indicating there was a pipeline explosion," Sumwalt said.

He said investigators would spend next week finding that outside caller and interviewing the CG control room operator. The NTSB would then construct a precise timeline to pinpoint when the Columbia Gas controller became aware of the blast.

Investigators also will look at procedures inside the control room and examine the training the operator received for his job.

Earlier Friday, Columbia Gas CEO Jimmy Staton issued an open letter to the community, saying "something went terribly wrong" and that his company is working tirelessly to find out what happened in Sissonville.

"I know the people of Kanawha County want answers, and rest assured, we will work with the National Transportation Safety Board to find the cause of this incident," Staton wrote. "Our primary responsibility is to ensure public safety through responsible operations.

"We are fully committed to supporting the NTSB investigation into this incident, and will take every step necessary to ensure the continued safety of our pipeline system."

Sumwalt said the NTSB's report would recommend steps that Columbia Gas should take to prevent another incident like this in West Virginia.

"We are very fortunate there was no fatalities but we also recognize that many had their lives turned upside down by this unfortunate event in your community," he said. "NTSB is dedicated to finding out what happened so it can never happen again and that's why we're here."

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.