Pieces of burnt insulation and debris were scattered over a wide area, and smoke and flames still rose from the site hours after the 1:40 p.m. blast in Plum, about 15 miles east of Pittsburgh.
"All of a sudden all the pictures blew off the walls; we heard a loud bang. It felt like an earthquake," said Diane Fulmer, 43, who lives two houses away. Fulmer said she ran outside and "all I could see was parts of houses flying in the air."
Richard Leith, 64, of Trafford, died at a Pittsburgh hospital, said John J. Smith, an investigator with the Allegheny County medical examiner's office.
Neighbor Lynn Celia told reporters she ran across the street right after the explosion and found Leith's granddaughter sitting in debris near the house next door. "I could see that she had been blown out of the house," Celia said.
Plum Borough Police Chief Frank Monaco said that the girl had a broken femur but that her injuries were apparently not life-threatening.
"That she wasn't killed is a miracle," he told the Pittsburgh Post-Gazette.
The girl was taken to Children's Hospital of Pittsburgh, where a spokesman said he did not have further details.
No other injuries were reported, but as many as 40 residents evacuated, and it was unclear when they would be allowed to return. An emergency shelter was set up at a nearby school.
Neighbor Nancy Wineland, 55, said she was cooking in her house when she heard the explosion, which cracked one of her windows.
"It was one huge, loud explosion, and the whole house shook," Wineland said. She said she ran through her house to check for possible damage before going outside, where she saw "flames and smoke and debris everywhere."
"The smoke was so thick, you couldn't see anything," she said. "You couldn't even breathe."
Elmore Lockley, a spokesman for the Dominion Peoples natural gas utility, told KDKA-TV the evidence points to a natural gas explosion, though what caused it was not immediately clear.
Crews were still searching for the cause but determined that gas lines on the street and adjacent streets were safe, Lockley said.
Lockley said there had been no reports of odors that might indicate a gas leak and no major construction in the area in recent months.