Meehan said investigators believed the woman, whose name was not immediately released, was alone in the house except for her dog, which also was killed. Investigators were still examining the rubble.
One firefighter suffered a minor injury, Meehan said.
Fire Marshall Steven Coan said his office was investigating the explosion. Meehan said the cause was suspected to be a natural gas leak.
Crews from the New England Gas Co, which serves Somerset, were in the area before the blast, he said. A company spokesman did not immediately return telephone and e-mail messages seeking comment.
Residents were evacuated from nearby homes to a temporary shelter at an elementary school. Meehan was not sure exactly how many people were relocated.
David M. Cucinotta and his wife had just finished dinner when they were felt the explosion around 6:30 p.m.
"There was just one big bang. We knew something was wrong," he said. "The house shook."
Stunned neighbors walked outside trying to figure out what happened.
Cucinotta's wife spotted a blaze on a nearby street, and he and a neighbor went to investigate. He saw heavy flames before arriving police officers began evacuating the block.
Helen Sullivan, who lives about 100 yards from the site of the blast, told The Boston Globe that there was "an overwhelming smell of gas" on the house's street earlier in the day.
Sullivan said the explosion sounded like "a very loud sonic boom." Another neighbor told WCVB-TV the blast sounded like a plane crash.
Natural gas leaks were reported in two recent house explosions. David Kupris, 45, was killed Dec. 17 in a blast in Scituate, and a Gloucester police officer was injured when his home exploded on Jan. 25.
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Associated Press writer Russell Contreras in Boston contributed to this report.
