Authorities say a Lowell apartment building fire that killed seven people last week was an accident caused by an electrical failure that began in a concealed space between two floors.

State Fire Marshal Stephen Coan said the pre-dawn fire Thursday burned undetected for a long period of time in the void space before a sudden rush of oxygen turned it into an inferno.

Coan said fireworks were in the building and detonated at some point, but did not play a role in starting the blaze.

The cause was announced Tuesday during a news conference at a fire station located just 100 yards from the scene of the fire.

Middlesex District Attorney Marian Ryan identified those who died as: Torn Sak, his longtime girlfriend, Ellen Vuong, and three of their five children, 38-year-old Tina Christakos and 72-year-old Robert Downs.