A natural gas leak caused a vacant house in Hempstead Village to explode Wednesday, the Nassau County fire marshal's office said Thursday.

One major clue: A hot water heater valve was in the "on" position, indicating it was drawing gas, said James Hickman, division supervisor of investigations.

"We know that there was a pilot light on . . . which probably provided a source of ignition," he said.

Other clues: The damage in the house was more extensive near the gas line and neighbors had complained of smelling gas since Monday, Hickman said.

"We took apart all the debris, and we were able to look at the appliances," he said. "Based on the damage of the house, all these things are consistent with a natural gas explosion."

The house at 180 Perry St. blew up just before 2:30 p.m., sending two people next door to the hospital with minor injuries and starting a small fire at the house on the other side.

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Investigators believe the explosion was accidental but must rule out other possibilities and piece together details before releasing more definitive answers. They want to pinpoint the leak source and find out why gas was flowing in a house that neighbors said had been vacant for months.

The property owner has been questioned, Hickman said, but details were not immediately available.

National Grid found no leaks in tests of its lines, which run from the street to the house, officials said, a clue that problems originated in the house.

Thursday, next-door neighbor Marie Guerrier still sounded shaken as she wondered what will happen to her home, which has a huge hole on the side. Her family now lives at the Marriott in Uniondale.

On Wednesday, she and daughter Benedice Auguste, 9, were by a living room window, listening to music on the computer, while other youngsters, 6-year-old twins and a boy, 2, were upstairs watching TV.

Suddenly, a blast opened a massive hole in the wall -- "That's where I was sitting," Guerrier said -- and flying glass cut her daughter on the temple.

Then a large cabinet in the upstairs hallway toppled down the steps, Guerrier said.

After a neighbor arrived to help, the mother clambered over books, and wooden and glass debris, and handed down her children, one at a time, to him.

Guerrier Thursday looked at her home of eight years, now off-limits because of safety concerns. But her daughter didn't need stitches and her younger children were not injured.

"Very scary," she said. "But everything's fine. So far so good."