Peter Knudson, from the National Transportation Safety Board, said 12 people were aboard the Pilatus PC-12 when it crashed about 12:30 p.m. Saturday, shortly after taking off from Chamberlain, about 140 miles west of Sioux Falls.
Knudson says nine people were killed and three were injured. The single-engine plane was bound for Idaho Falls, Idaho.
Media reports say Chamberlain and central south-central South Dakota were under a winter storm warning at the time of the crash.
Knudson says weather will be among several factors NTSB investigators will review, but no cause has yet been determined. He says inclement weather is making travel to the site difficult.
No further information was immediately available.