CHARLESTON, W.Va. -- It's still not clear how NiSource Gas Transmission and Storage officials were alerted to a massive explosion at a 20-inch-diameter transmission line near Sissonville, investigators from the National Transportation Safety Board said Thursday.

Investigators said previously that no alarm went off Tuesday afternoon at the gas company's control room that would notify its personnel the pipeline had ruptured near Columbia Gas Transmission's Lanham Compressor Station at Rocky Fork.

The blast and resulting fires destroyed about five homes and engulfed a large section of Interstate 77. No one was killed but several people were treated for minor injuries.

NTSB member Robert Sumwalt told reporters Thursday evening at the Holiday Inn in South Charleston that investigators are interviewing company personnel and are reviewing control room records to determine why the alarms didn't go off.

Investigators discovered that pressure measured within the pipe dropped significantly at about the time the first 911 call came in about the explosion.

"All we are saying at this point is, because we haven't gotten a detailed look at the records, we just know there was a pressure decrease [but] we don't know the magnitude of that decrease," he said. "We want to find out how much the decrease was and if it reached the level of the alarms, why the alarms did not go off."

NTSB investigators will focus on whether NiSource officials acted adequately in shutting down gas to the ruptured line.

Based on preliminary data, Sumwalt said the explosion occurred at 12:41 p.m. and gas to the pipeline was shut off at about 1:45 p.m., 64 minutes later.

A 10-member team of NTSB investigators visited the crater site Thursday for a preliminary examination of the ruptured pipe, Sumwalt said.

It appears 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, he said.

An area along the bottom of the pipe, running about 6 feet long, was measured at less than one-tenth of an inch thick, he said. This thinning indicates that some segments along the pipe wall were about 30 percent thinner than required.

"There are many things that can cause pipe wall thickness to deteriorate and that is exactly what we will be looking at . . . ," Sumwalt said. "What caused this pipe wall to become deteriorated? What caused it and what finally led to the actual rupture and explosion?"

Investigators said previously that no alarm went off Tuesday afternoon at the gas company's control room that would notify its personnel the pipeline had ruptured near Columbia Gas Transmission's Lanham Compressor Station at Rocky Fork.

The blast and resulting fires destroyed about five homes and engulfed a large section of Interstate 77. No one was killed but several people were treated for minor injuries.

NTSB member Robert Sumwalt told reporters Thursday evening at the Holiday Inn in South Charleston that investigators are interviewing company personnel and are reviewing control room records to determine why the alarms didn't go off.

Investigators discovered that pressure measured within the pipe dropped significantly at about the time the first 911 call came in about the explosion.

"All we are saying at this point is, because we haven't gotten a detailed look at the records, we just know there was a pressure decrease [but] we don't know the magnitude of that decrease," he said. "We want to find out how much the decrease was and if it reached the level of the alarms, why the alarms did not go off."

NTSB investigators will focus on whether NiSource officials acted adequately in shutting down gas to the ruptured line.

Based on preliminary data, Sumwalt said the explosion occurred at 12:41 p.m. and gas to the pipeline was shut off at about 1:45 p.m., 64 minutes later.

A 10-member team of NTSB investigators visited the crater site Thursday for a preliminary examination of the ruptured pipe, Sumwalt said.

It appears 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, he said.

An area along the bottom of the pipe, running about 6 feet long, was measured at less than one-tenth of an inch thick, he said. This thinning indicates that some segments along the pipe wall were about 30 percent thinner than required.

"There are many things that can cause pipe wall thickness to deteriorate and that is exactly what we will be looking at . . . ," Sumwalt said. "What caused this pipe wall to become deteriorated? What caused it and what finally led to the actual rupture and explosion?"

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.