The roof of the home in Putnam, in northeastern Connecticut, was engulfed in flames when firefighters arrived at about 1 a.m. Tuesday, and it later collapsed into the third floor, where the children were, authorities said. The fire wasn't under control until about 6:30 a.m.
The bodies of the two dead children were found Tuesday afternoon. State police provided their ages, but their names and other details weren't provided.
Neighbor Brian Vaiciulis said the sound of a truck engine woke him up at about 1:20 a.m.
"I got up, and I saw the lights on the fire trucks," said Vaiciulis, who didn't know any residents of the house that caught fire. "Then I saw the flames over the house next door. They were pretty high."
With the children initially unaccounted for, police Chief Rick Hayes said that officials were not able to immediately enter the third-floor area for safety reasons. Putnam Fire Marshal Norm Perron said the top floor was stabilized at about noon.
Hayes said the siblings' mother was one of the injured adults and he was told she is pregnant with twins. She was being treated for undisclosed injuries at a hospital in nearby Worcester, Mass.
The state medical examiner will perform autopsies on the bodies of the two children. State police detectives are conducting an untimely death investigation "just to clarify" the circumstances of the children's deaths, Lt. J. Paul Vance said.
Officials said the three other adults were treated for smoke inhalation at a local hospital.
Fire investigators were trying to determine the cause of the blaze.
Officials believe there were five apartments in the Victorian-style house, which was built in 1867 and had nearly 4,500 square feet of living space, according to town records. The house was gutted by the fire.
Records show the property's owner is Eastern Connecticut Contractors LLC. The Associated Press left messages Tuesday for officials with the company and the firm's attorney.
Martin Renaud, who lives across the street from the house, said he awoke at about 12:45 a.m. and saw the house on fire.
"I saw the windows. They were flashing orange," he said.
Renaud added that the house lasted "through days of heating with coal and heating with wood and then it burns in 2013."
Associated Press writer Dave Collins in Hartford contributed to this report.
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