SOUTH CHARLESTON, W.Va. -- Officials began surveying the ruptured gas pipeline that caused a massive explosion Tuesday in Sissonville -- the first step in what they promised would be an extensive investigation into what caused the blast and how it could've been prevented.Investigators from the National Transportation Safety Board traveled to the site near Columbia Gas Transmission's Lanham Compressor Station at Rocky Fork, where an explosion occurred on a 20-inch-diameter transmission line Tuesday.Several people received only minor injures, but the blast and resulting fires destroyed approximately five homes and engulfed a large section of Interstate 77.NTSB member Robert Sumwalt said a preliminary investigation hasn't determined yet if the pipe is directly attached into the Lanham Compressor Station or if that pipe is related to a similar explosion there in 2002.No one interviewed so far, including residents and emergency responders, reported smelling gas fumes anytime before the incident, Sumwalt said.He said no alarms went off at the gas company's control station, either.Ravi Chhatre, NTSB's lead investigator on the incident, and Sumwalt said they would eventually release a report about the blast and a timeline leading up. Sumwalt and Chhatre spoke during a briefing Wednesday evening at the Holiday Inn in South Charleston.The NTSB is an independent government agency that investigates civil transportation accidents. The board has no regulatory or enforcement authority, but instead issues reports that detail why accidents occur and recommends changes that industry and regulatory agencies could make to avoid future incidents.The heart of the investigation, Sumwalt said, would focus on the gas company's response time in shutting off gas to the pipeline after the explosion.Sumwalt described the pipeline that blew up during a briefing Tuesday night as being an interstate transmission line owned by NiSource subsidiary Columbia Natural Gas Transmission. By Wednesday night, Sumwalt said the company had originally told agency officials it was a 30-inch line, but NTSB investigators later confirmed it was actually a 20-inch pipe.On Wednesday, Chevalier Mayes, communications manager for pipeline operator NiSource Gas Transmission and Storage, did not respond to questions about the age of the pipeline and requests for details of any recent examinations of it by the company. "Much of the information you inquired about is being assessed as part of the ongoing investigation," Mayes said in an email message. "We'll share more details as we can."Mayes also declined to describe what sort of shutoff systems the company had in place at the pipeline.Based on preliminary data, Sumwalt said the explosion occurred at approximately 12:41 p.m. and gas to the pipeline was shut off at about 1:45 p.m., 64 minutes later.Sumwalt compared that shutdown time to similar explosions NTSB has investigated. They have looked at pipelines that were shut off almost instantaneously and others that took more than an hour. All responses vary, he said."Part of our investigation will look to see if this pipeline was shut down at a reasonably early fashion," he said.Following a September 2010 natural gas transmission pipeline explosion that killed eight people in San Bruno, Calif., the National Transportation Safety Board harshly criticized the emergency response efforts of the pipeline operator, Pacific Gas and Electric Co. 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."The investigation into the Sissonville explosion would continue Thursday as officials extract the dislocated pipe to send to the NTSB's lab in Washington, D.C. for a "detailed material examination."So far, investigators have not determined what material the pipe is made from because it's too early to definitely say."To tell you the truth, it's not as easy of a process as you may think to know exactly what the pipe is and the attributes of that pipe," he said.Investigators also would extract about 10 feet of undamaged pipe nearby to see if it also presented an explosion risk.The pipeline 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.The site of the investigation also remains too hazardous. Investigators are not permitted to use cellphones and must be accompanied by someone with a gas meter."It could be weeks before we know the cause of the explosion," state Homeland Security and Emergency Management Director Jimmy Gianato said Wednesday morning.Gianato said the first report of the explosion incorrectly stated that the Cedar Ridge Center nursing home had blown up, "so we were expecting a lot of casualties."It turned out that Cedar Ridge was not damaged in the blast, though it did lose power and phone service, and had to make use of a generator and cellphones, although several homes in the vicinity were destroyed or damaged."It's a miracle no one happened to be home in those places when the explosion happened," Gianato said. "And with the volume of traffic that travels on the interstate, we were very fortunate that no cars were damaged on I-77 or Route 21.""After this, I think we will see more inspection and testing of pipelines going on," Gov. Earl Ray Tomblin said.Gianato added: "We're all interested in seeing what we can do to make sure something like this doesn't happen again."Sumwalt said investigators would soon be listening to 911 calls and firefighter dispatches after the blast to create a precise timeline. They also plan to talk to everyone within NiSource Gas Transmission and Storage who controlled and maintained the pipe.Anyone with information about the pipe or the explosion is asked to contact NTSB by email at firstname.lastname@example.org.Sumwalt said he would hold another briefing Thursday at 5 p.m. at the Holiday Inn in South Charleston.Staff writers Rusty Marks, Kathryn Gregory and Ken Ward Jr. contributed. Reach Travis Crum at email@example.com or 304-348-5163.