HOMER, ALASKA, NOV. 28 -- Investigators looking into the crash of a heavily loaded commuter plane that killed 18 people have turned their attention to the critical question of how that weight was distributed, a federal official said.

The Ryan Air twin-engine turboprop, possibly coated with a heavy layer of ice, smashed through an airport fence and belly-flopped onto snow-covered ground short of the runway as it tried to land Monday evening at the Homer airport. Three of those aboard survived.

The plane, carrying its full capacity of 19 passengers and two crew members, had an estimated 16,100 pounds of cargo, its maximum allowable weight, said Barry Trotter, head of the National Transportation Safety Board team investigating the crash.

"Because of estimates on the people still in the hospital, we're going to be over or under 100 pounds," he said Friday. "The aircraft was designed to fly with that much weight. It's where it's loaded in the aircraft that makes it critical.

"If you put it too far in the tail, it tends to be tail-heavy and make it more critical flying," he added.

The rear of the fuselage appears to have struck the ground first, about eight feet outside the airport perimeter fence. The plane then plowed through the 8-foot chain link fence, catching its left wing on a fence pole and sliding about 170 feet, Trotter said.

Photographs and accounts from rescuers indicate that there was a quarter-inch of ice on the plane's rudder and rear stabilizers, but no ice on the wings. That amount would not be significant, but investigators are not ruling out the possibility of heavier icing that went undetected, Trotter said.

Water and foam sprayed by firefighters could have washed away ice that may have coated the plane during flight, he said. A heavy coating of ice could have brought the heavily laden airplane over its allowable landing weight.