At a news conference Tuesday, State Fire Marshal Steve Coan said the blaze began in a crawl space between the wood-frame building's second and third floors. He said the fire spread undetected in that area and exploded when it burned through the floor, getting a new source of oxygen.
Coan said that fireworks stored in at least one apartment did not cause the fire. One of the victims had posted a picture on Facebook a few days earlier of his fireworks stash.
The alarm system was working properly, but may have become inoperative because of the fire, Coan said. He said inspectors did not find any code violations.
The fire, which broke out in the early morning last Thursday, killed Torn Sak and Ellen Vuong, their three children, Anthony, 12, Ryan, 8, and Sayuri, 7, and two other adults. Coan said the fire was the deadliest in Massachusetts in 20 years.
Ironically, last week a judge ordered a new trial for Victor Rosario, who has spent more than three decades in prison for a fire that killed eight in Lowell. Rosario's lawyers said the investigation into the fire wrongly determined it was arson.