ANDOVER - Dozens of explosions, apparently triggered by a natural gas pipeline rupture, rocked three communities near Boston on Thursday, injuring at least six people and prompting the evacuation of hundreds, local officials said.The blasts left scores of homes and other buildings demolished or engulfed in flames as firefighters from some 50 departments scurried for hours from one blaze to another and utility crews rushed to shut off gas and electricity in the area to prevent further ignitions.Fire investigators suspected "over-pressurization of a gas main" belonging to Columbia Gas of Massachusetts led to the series of explosions and fires, Andover Fire Chief Michael Mansfield said at a news conference.Columbia Gas, a unit of NiSource Inc, had announced earlier in the day that it would be upgrading gas lines in neighborhoods across the state, including the area where the explosions occurred. But it was not immediately known whether any work was being done in those communities at the time."Columbia Gas is investigating what happened on its system today," NiSource spokesman Ken Stammen said.The US Department of Transportation's Pipeline and Hazardous Materials Safety Administration (PHMSA) said it was dispatching a team to support the state's emergency response efforts.The former head of that agency, Brigham McCown, called the series of gas explosions "unprecedented, at least in recent memory.""I can't think of a series of natural gas-related incidents like this," McCown, who led PHMSA under then-President George W. Bush, told Reuters by telephone. "We have had similar issues on a much smaller scale."He said the National Transportation Safety Board was also sending a team headed by its chairman, Robert Sumwalt, to investigate the disaster.Live TV images showed firefighters battling blazes in the former mill towns of Lawrence, Andover and North Andover, about 25 miles (40 km) north of the Massachusetts capital.The streets were darkened after nightfall as power was cut off to the area as a precaution.'OVERWHELMING EVENT'At least three people, including one firefighter, were hurt in Andover, the town said in a statement. Six people were being treated at Lawrence General Hospital, spokeswoman Jill McDonald Halsey said. It was not immediately clear if any of those being treated at that hospital were from Andover.The Boston affiliate of CBS News reported that two of the six patients at Lawrence hospital were in critical condition."This has been an overwhelming event," Mansfield told reporters, saying that fire companies responding to blazes were putting one house fire out only to discover another house on fire as they finished their work. "This event is not over tonight. This event will probably go on."Massachusetts State Police said a total of 70 fires, explosions or investigations of gas odor had been reported. Some 50 fire departments responded to the emergency, said John McArdle, fire chief of Plaistow, New Hampshire, which sent a tower ladder and engine crew to the scene.North Andover resident Laurie Williams, 55, said she drove home from work to find police and fire vehicles blocking streets near her home, and learned that three buildings nearby were on fire."It was very scary," Williams told Reuters by telephone, describing smoke visible to her on the skyline. "My first thought was this is a gas explosion."State police urged residents served by Columbia Gas to evacuate their homes. Lawrence Mayor Dan Rivera said residents in the southern part of his city had been asked to leave their homes whether or not they were Columbia Gas customers, due to planned power cuts.Gas utilities have come under heightened scrutiny in recent years for an aging network of pipelines that critics say are not being properly inspected and maintained, posing a growing risk to public safety.Friday's conflagrations came six years after Columbia Gas accepted blame for a gas explosion in Springfield, Massachusetts that injured 17 people and caused an estimated $1.3 million in property damageThat occurred when a Columbia Gas technician called to investigate a gas odor at a nightclub accidentally punctured a line, the state Department of Public Utilities said in a report.