The NTSB also said it appears the pilot was working the radio rather than flying the plane, indicating the co-pilot was in charge.
Wellstone and seven others died Oct. 25 when their plane went down on approach to Eveleth, Minn., where Wellstone was to attend a friend's funeral. Killed with Wellstone were his wife, Sheila, their daughter, Marcia Wellstone Markuson; campaign aides Will McLaughlin, Tom Lapic and Mary McEvoy and pilots Richard Conry and Michael Guess.
The NTSB said Conry and Guess had received two flight service weather briefings before taking off.
"Specialists at the National Center for Atmospheric Research in Boulder, Colo., are assisting the NTSB weather group in its efforts to more accurately define the icing conditions that existed along the accident flight's route," the NTSB said.
Investigators are trying to determine whether the twin-engine plane's de-icing boots were functioning as light snow and freezing rain fell. The investigation was complicated by the plane's lack of a cockpit voice recorder.
An examination of the plane's engines by Pratt and Whitney of Canada and its propellers at Hartzell company facilities in Ohio indicated all were operating properly at the time of impact.
The NTSB said toxicological tests indicated neither pilot was impaired and maintenance records indicate the ill-fated plane had been properly serviced.
Next week, investigators plan to use a flight simulator at a Flight Safety International facility in Wichita, Kan., to study the plane's approach profile. An examination of the radar equipment at Eveleth indicates the signal was slightly out of kilter for an instrument landing. The plane began to drift off course at least 2 minutes before it crashed.