The head of the National Transportation Safety Board said Saturday that investigators did not suspect foul play or nefarious intent in a series of natural gas pipeline explosions Thursday north of Boston that killed one person and set dozens of houses on fire.Robert Sumwalt, chairman of the NTSB, told reporters that agency officials were not on the scene to determine when people could go back to their homes. He said his agents planned to do a "deep dive" into the cause of the accident, which could take as long as two years.Sumwalt said NTSB officials expected to be in Massachusetts for seven to 10 days, documenting physical damage and collecting records and perishable evidence.He said safety officials wanted to look at what the controller of the pipelines in question knew before the explosions occurred. He said investigators would also examine the nature of any complaints about gas odors in the area in the days leading up to the explosions, and any records of failures, repairs or inspections in the recent past.Meanwhile, the natural gas company in charge of the faulty lines was struggling to explain its early response to the crisis, which officials said injured about two dozen people and damaged 60 to 80 homes.The head of Columbia Gas, Stephen Bryant, on Friday offered his "sincere, deepest condolences" to the family of Leonel Rondon, the 18-year-old who was killed Thursday when a collapsing chimney fell on his car after a house blew up.Response defendedAt a news conference, Bryant sought to counter criticism that the company had not responded quickly or clearly enough to the crisis. "Generally, I would say we have advanced this as rapidly as it could possibly be advanced," he said. "I don't think anybody else managing this would have been farther down the road than we are at this moment." But residents and local officials have complained that Columbia Gas has not responded quickly, clearly or effectively to the crisis. On Friday, Massachusetts Gov. Charlie Baker declared a state of emergency for the Merrimack Valley region and said he was assigning a rival energy company to take charge of recovery."The follow-through just wasn't there," he told The Boston Globe. "We need to get on with this."Officials said 8,000 people were forced to evacuate their homes while emergency workers checked each dwelling serviced by Columbia Gas to shut off the gas lines. Electrical power to those homes was also shut off to avoid a spark that could ignite any leaked gas and cause a new disaster.Four hundred people went to shelters after Massachusetts State Police urged them to evacuate their homes. Baker said it might be days or weeks before those displaced could return to their homes.Preliminary investigations indicated excess pressure in the gas lines might have caused the blasts. Before Thursday's disaster, Columbia Gas had notified its customers that it would be upgrading gas lines in neighborhoods across the state.