Security tape is strung inside the Holy Cross Cemetery after a single-engine plane crashed in an area just south of the Bert Mooney Airport in Butte, Mont., on Sunday. (Walter Hinick/Associated Press)Speculation about the plane that crashed Sunday in Montana shifted to ice on the wings after it became less likely that overloading was to blame, given that half of the 14 victims were small children.

While descending in preparation for landing at the Butte, Mont., airport, the single-engine turboprop plane passed through a layer of air at about 460 metres that was conducive to icing because the temperatures were below freezing and the air, according to accuweather.com "had 100 per cent relative humidity, or was saturated."

Safety experts said similar icing conditions existed when a Continental Airlines twin-engine turboprop crashed into a home near Buffalo Niagara International Airport in New York last month, killing 50.

"It's Buffalo all over again, or it could be," said John Goglia, a former member of the U.S. National Transportation Safety Board. "Icing, given those conditions, is certainly going to be high on the list of things to look at for the investigators."

Mark Rosenker, acting chairman of the U.S. NTSB, told reporters Monday in Montana that investigators will look at icing on the wings as a factor.

"We will be looking at everything as it relates to the weather," he said.

Hours after Sunday's crash, federal investigators said they'd will look into whether aircraft bound for a ski resort was overloaded when it nosedived into the ground and killed 14 people on board, seven of whom were children.

Rosenker said the plane was designed to carry only 11 people, including two pilots. He surmised that some of the children may have been small enough to sit on adults' laps during the flight.

"It will take us a while to understand," he told a news conference. "We have to get the weights of all the passengers, we have to get the weight of the fuel, all of the luggage."

Federal officials confirmed that seven adults, including the pilot, and seven children died when the aircraft nosedived into the Holy Cross Cemetery about 150 metres short of Bert Mooney Airport. There were two four-year-olds on board, along with children aged one, three, five, seven and nine, reports said.

The flight was scheduled to fly to Bozeman, Mont., from Oroville, Calif., but pilot Buddy Summerfield had spoken with air traffic controllers to inform them that he was diverting to Butte, Rosenker said. During that conversation, Summerfield, a former air force pilot who logged 2,000 hours flying that type of aircraft, didn't indicate that he was experiencing any problems.

Officials have not yet indicated why the pilot decided to divert the flight, but that should not have created problems because it was the original backup plan.

At the time of the crash, it was partly cloudy, visibility was 16 kilometres and winds were from the northwest around 16 km/h, according to hourly information from the National Weather Service.

NTSB investigators arrived in Butte on Sunday night and began combing through the rubble of the Pilatus PC-12 aircraft at the cemetery at daybreak Monday.

Officials said it is not yet clear what caused the crash and the investigation will be complicated by the absence of a cockpit voice recorder or a flight data recorder, which isn't required for smaller aircraft that don't fly commercial passengers, like airlines and charter services. They expect to recover all the aircraft wreckage by Friday, but answers could be months away.

On way to ski resort

Irving M. (Bud) Feldkamp, who lost two daughters and their families in the crash, said all were going to the Yellowstone Club for a skiing vacation. The club, near Yellowstone National Park, is a millionaires-only resort that counts former U.S. vice-president Dan Quayle and Microsoft founder Bill Gates among its members.

"We were going on a vacation with all the grandkids," said Feldkamp. "They were all excited about skiing."

The 2001 Swiss-manufactured Pilatus PC-12 was registered to Eagle Cap Leasing Inc. in Enterprise, Ore., said FAA spokesman Mike Fergus. He didn't know who was operating the plane.

NTSB investigator Kristi Dunks declined to comment at a news conference on Sunday night if there had been a distress call from the pilot.

Witnesses have reported seeing the plane jerk left before apparently losing control.

'Nothing left of it'

Butte resident Steve Guidoni said he was driving by the cemetery and saw the plane hit the ground. He stopped and ran over to see if he could help.

"It smelled like diesel fuel to me," said Guidoni, 61. "There was nothing left of it. It just went straight into the ground. I went over there to try to help. I thought maybe I would pull someone out of the fire."

Guidoni said he saw luggage and seat cushions, but no bodies. He said the biggest piece of the plane was the size of a kitchen table.

"You wouldn't even know a plane was there," he said.

The crash is the fourth major plane accident in the U.S. in slightly more than three months.

With files from the Associated Press