CHARLESTON, W.Va. -- Controllers at NiSource received 16 "pressure-drop alerts" between the time a Sissonville natural gas pipeline blew up and a phone call from another company reporting the incident to them, according to a preliminary federal report issued Wednesday.National Transportation Safety Board investigators plan to look into what exactly caused those alerts and how well NiSource employees responded to them, a board spokesman said.The day after the explosion, NTSB officials had said that the NiSource control room received no "alarms" at the time of the blast.Peter Knudson, an NTSB spokesman, said Wednesday those statements referred to more serious "critical alarms," and that the "pressure-drop alerts" were among the many -- and mostly routine -- notifications that control-room personnel receive more frequently. The alerts came after the explosion, Knudson said, but prior to NiSource learning of the explosion."It indicates there were some pressure drops," Knudson said. "What [the series of alerts] means in the big scheme of things, that's part of our investigation -- when did they come and how were they handled?"Information about the alerts was among the few tidbits of new details contained in the preliminary NTSB report on the Dec. 11 incident.The NTSB has said the blast occurred on a 20-inch-diameter natural gas transmission line. Several people received minor injuries, several homes were destroyed and the ensuing fire engulfed and damaged a large section of Interstate 77 north of Charleston.The first notification to NiSource came from a Cabot Oil and Gas controller who had received a report of a rupture and fire from a field technician who was near the accident location. NTSB officials estimate the explosion occurred at 12:41 p.m., and the call to NiSource from Cabot at 12:53 p.m.Officials from NiSource subsidiary Columbia Gas Transmission have generally refused to answer questions about the incident.Just minutes after the NTSB posted its preliminary report online Wednesday afternoon, NiSource Columbia Gas issued a statement says the company would "implement appropriate measures" based on recommendations the NTSB eventually makes when its investigation is complete.The two-page preliminary NTSB report confirmed that the pipeline segment that exploded was installed in 1967.One other new piece of information was that about 20 feet of pipe that was ejected by the explosion landed about 40 feet from its original location, not 50 feet as NTSB investigators had originally reported.The pipeline was not equipped with automatic or remote shutoff valves. The NTSB had said the explosion occurred at 12:41 p.m., and that NiSource crews were not able to manually shut off the flow of gas to the fire until 1:45 p.m. An order from the U.S. Department of Transportation said the explosion occurred at 12:43 p.m., and that one end of the pipeline was shut off manually at 1:20 p.m. and the other at 1:40 p.m.Reach Ken Ward Jr. at firstname.lastname@example.org or 304-348-1702.