Gloucester City, N.J. ? Investigators may never learn what started the blaze that killed three little girls and three of the firefighters who tried to save them, an official said Friday.

The utter destruction of the twin wood homes Thursday will make it more difficult and perhaps impossible to determine the fire’s cause, said Mark Chait, an agent with the federal Bureau of Alcohol, Tobacco and Firearms in Philadelphia.

Alexandra Slack, 5, center, and twins Claudia and Colletta, 3, died Thursday in a fire in Gloucester City, N.J.

Authorities said, however, that there was no evidence the fire had been set.

The blaze turned Thursday’s holiday into a mourning day for the Philadelphia suburb of 11,500.

One of the men who died, Thomas G. Stewart III, had used a fire truck’s loudspeaker system to propose to Danielle Ruggiero in front of a cheering crowd gathered at a high school football stadium to watch fireworks Wednesday night.

About 12 hours later, Stewart was called to the fire in a 2 1/2-story duplex where 3-year-old twins, Claudia and Colletta Slack, and their 5-year-old sister, Alexandra, were trapped.

Before rescuers could reach the girls, the roof collapsed. A neighbor said the cave-in looked like a falling deck of cards.

The little girls were killed, along with Stewart, Mount Ephraim Fire Chief James E. Sylvester and Deputy Chief John D. West. Five other firefighters were pulled out alive and suffered only minor injuries.

Neighbors said their mother, Katia Williamson, 24, rushed back into the house to try to save them. She was listed in critical but stable condition Friday at a Chester, Pa., hospital.

Their father, Frank Slack, 27, was treated for smoke inhalation, said Greg Reinert, a spokesman for the Camden County prosecutor’s office.

The ATF doesn’t normally investigate residential fires. Chait said the agency joined the probe partly because of the three firefighters’ deaths, and because of a personal connection: West had an ATF-trained search dog.