Gloucester City, N.J. ? Fire broke out in a 2 1/2-story duplex Thursday, killing 3-year-old twins, their 5-year-old sister and three firefighters trying to rescue them, authorities said.

One of the three firefighters had proposed to his girlfriend at a Fourth of July fireworks display only 12 hours earlier.

Fireman search through debris for victims after the roof of a multi-story duplex that caught fire collapsed in Gloucester City, N.J. The fire Thursday killed 3-year-old twins, their 5-year-old sister and three firefighters who came to their rescue, authorities said.

The blaze was being investigated as a criminal incident, although there was no evidence it had been set, authorities said.

The three firefighters who died had been looking for the three young sisters when the roof collapsed, authorities said. Five other firefighters also were trapped but were dug out by comrades, Acting Camden County Prosecutor James Lynch said.

“It was like a deck of cards falling down,” said neighbor Monique Gagliardi as she watched rescue workers remove a child’s body from the debris. “It was horrible.”

The fire was extinguished within about five hours, and rescuers used backhoes to clear debris while they searched for victims.

Two Mount Ephraim volunteer firefighters, Chief James Sylvester and Deputy Chief John West, were killed alongside Thomas Stewart III, who had used the public address system on a fire truck Wednesday night to propose to his finance.

“They were dedicated firemen and they died doing what they loved,” said Anthony Stagliano, a spokesman for the Gloucester City Fire Department.

The girls’ mother was hospitalized in critical but stable condition. Gagliardi said the woman ran back into the house to try to save her daughters. Their father was treated for a minor case of smoke inhalation, said Greg Reinert, a spokesman for the prosecutor’s office.

Eight firefighters were treated at a hospital for minor injuries and released.

Gov. James McGreevey stopped at the hospital on his way to the fire scene. He said the three firefighters made “the supreme sacrifice,” and he promised the state would pay for their children’s college education.

Independence Day festivities in the town of 11,000 residents were canceled because of the fire.

Gloucester City is about five miles south of Philadelphia.