A fire engulfed an abandoned house in West Philadelphia early Sunday, killing four children and a woman who had apparently taken refuge there from the cold, authorities said.

Firefighters found the victims unresponsive on a bed inside the house's boarded-up second floor, said Fire Commissioner Lloyd Ayers. Rescuers rushed to pass the victims out of windows and paramedics tried to revive them, he said.

Officials were not immediately able to identify the victims or determine their ages. Ayers said they included a young woman who was apparently the children's mother, a girl and three boys. The children all appeared to be under age 10 and the youngest was in diapers, he said.

"It's a tough day for firefighters here," Ayers said.

The fire started at around 3:20 a.m., Ayers said. Firefighters discovered the victims after pulling the plywood off the building to try to gain access, he said.

Investigators were working to determine who owned the building and what caused the fire.

Ayers said the electrical wiring was rigged dangerously and the house was choked with debris. He said the victims appeared to have taken refuge there from the cold.

"It was just in a shambles, no place where human beings should be trying to live," he said. "It was just a family trying to live in this cold."