State Fire Marshal Stephen Coan said the pre-dawn fire burned undetected Thursday in the void space for an undetermined period before a rush of oxygen turned it into an inferno. Coan said fireworks stored in the building detonated at some point but didn't play a role in starting the blaze.
The cause of the fire was announced during a news conference at a Lowell fire station located 100 yards from the scene of the blaze.
Middlesex District Attorney Marian Ryan identified the victims as Torn Sak, his longtime girlfriend, Ellen Vuong, three of their five children, 38-year-old Tina Christakos and 72-year-old Robert Downs. Christakos and Downs were friends and roommates who lived in a separate apartment in the building.
Coan said an investigation found that the fire started in a void space between the second and third floors of the building. He said the fire traveled undetected along the length of the building before breaking through the concealed space and then getting a sudden rush of oxygen.
"It quickly became an inferno with intense heat, and thick, black, choking smoke," he said.
Coan said the fire alarm system within the building was working, although the electrical failure appears to have at least partially disabled it. He said some of the 49 people who escaped the fire said they heard horn alarms in the building's hallways, but they were muffled and intermittent. Residents also had hard-wired smoke alarms with battery backups in their units. Coan said the majority of residents who were interviewed said they had heard one or both of the alarm systems.
Coan also said investigators "did not find that to be a troubled building" and had not cited the landlord for code violations.