Federal investigators will focus on a suspected leak in an underground natural gas pipeline when they begin their probe next week into a deadly Christmas Eve explosion that leveled a house in a Sacramento suburb.

The National Transportation Safety Board spokeswoman Bridget Serchak confirmed that a pipeline investigator would begin work next week. The agency was notified because the gas leak is suspected to originate from a pipeline buried beneath the roadway.

Pacific Gas & Electric Co., the area's natural gas utility said it would share results of its preliminary investigation with federal authorities.

The blast destroyed the house in Rancho Cordova, killing a 72-year-old man and severely injuring his daughter and granddaughter. Two homes on either side were also damaged.

One neighbor who saw one of the badly burned victims being thrown from the leveled house questioned why gas service wasn't shut off to the cul-de-sac or residents evacuated, especially since utility workers had been notified hours earlier.

"It's a tragedy because people had called and reported it," said Gigi Lopez, who lived next door and began smelling a foul odor the night before. "They called, but to no avail. This could have been prevented had (the utility) not waited until the next day."

Brian Swanson, a spokesman for Pacific Gas & Electric Co., the utility that supplies natural gas to the area, said a worker who was searching for the leak found no excessive gas flow at the house and began alerting residents nearby. The gas company recommends people leave a building if they smell gas and call for help once outside.

"It wasn't that type of an emergency" to warrant an evacuation, Swanson said. "If we did find there was excessive gas flow, we would turn the gas off to the home at the meter."

Neighbors said Friday that the three-bedroom house had a gas leak over a year ago and that at least three neighbors had complained about a foul odor as early as Tuesday night.

Swanson said the utility's records indicate it received the first call about a gas leak Wednesday morning.

Neighbors described the scene as a war zone as glass and wood rained down on the cul-de-sac while the remainder of the house burned.

"I literally saw my neighbor being thrown. Her hair looked like plastic ... My neighbor said she helped her from the grass and there was skin hanging off her. That was how bad she was burned," Lopez said.

She said neighbors found Wilbert "Bill" Paana crying for help beneath debris. One of the people who helped, Joe Helton, had described the older man as badly wounded and covered in blood.

Paana, 72, died in the hospital late Wednesday night after suffering severe abdominal injuries, according to the Sacramento County Coroner's Office.

The other burn victims include 44-year-old Kim Dickson, who remained in critical condition at the University of California, Davis Regional Burn Center, and her daughter, 17-year-old Sunny Dickson, who was being treated at Shriners Hospital for Children.

A neighbor, a utility worker and a firefighter also were hurt but were released from area hospitals after being treated for minor injuries.