Federal investigators believe icing may be a cause of the plane crash that killed Sen. Paul Wellstone and seven others after determining the engines and propellers were working properly.

The National Transportation Safety Board issued a progress report Tuesday as part of its investigation into why the chartered King Air A100 plane crashed and burned about 2 miles from the Eveleth-Virginaids and human error, fatigue or disorientation.

Wellstone, D-Minn., was headed to the funeral of a state representative's father when the plane crashed in freezing rain and light snow. Wellstone's wife, Sheila; their 33-year-old daughter, Marcia Wellstone Markuson; and five others also were killed.

The NTSB said specialists at the National Center for Atmospheric Research in Boulder, Colo., are working with the safety board to determine the icing conditions that existed along the flight's route.

Paul Czysz, professor emeritus of aerospace engineering at Saint Louis University, said it only takes an eighth of an inch of ice on the wing's leading edge to disrupt the flow of air. That can cause the wing to lose its lift and the plane to suddenly change course.

Investigators say Wellstone's plane made an abrupt right turn as it approached the runway. The weather conditions made it difficult for the pilot to recover, Czysz said.

"It was a gray, misty day," he said. "When you fly in visually degraded conditions, it's really tough to know how fast you're going and how you're oriented."

Safety investigators said next week at a facility in Wichita, Kan., they will try to simulate the flight and weather conditions to learn more about what happened just prior to the crash.

Bill Waldock, professor of safety science at Embry-Riddle Aeronautical University in Prescott, Ariz., said it's rare for just one factor to cause a crash.

"It's a combination of things," Waldock said. "You put them together, you end up with an accident."

Toxicological reports revealed no evidence of drug or alcohol abuse by pilot Richard Conry or the first officer, Michael Guess. The NTSB said based on radio transmissions it believes Conry was at the controls at the time of the crash.

The report said investigators have documented the crew's activities for three days before the flight through interviews with Conry's wife and Guess's fiancee. They still are reviewing employment and medical records.

Conry, 55, had worked long hours the day before the crash, flying roundtrip between St. Paul, Minn., and Bismarck, N.D., from 3 a.m. to 9:30 a.m. and then working a four-hour shift at his second job as a nurse. That shift ended around 9:30 p.m., roughly 12 hours before Wellstone's plane took off.

However, federal regulations require only a 10-hour off-duty period between scheduled flights.

Since the crash it was learned Conry exaggerated his flying experience to get a job at Aviation Charter, which operated the plane Wellstone was on. Aviation Charter has said Conry was a qualified pilot who performed well during his year and a half at the company.