The six-seat, single-engine plane was travelling from Kenosha to Menomonie when it crashed in a wooded area near Mirror Lake State Park, not far from the popular tourist destination of Wisconsin Dells, the Sauk County Sheriff's Department said.
The National Transportation Safety Board said the pilot told an air traffic controller about icing on the wings of the Piper PA-46 Malibu as it flew at 7315m and requested permission to descend.
Ice build-up on the wings is a major aviation hazard because even a thin layer of ice can make it difficult for a plane to remain airborne.
Air traffic control approved the descent, and the plane began a right descending turn that increased in rate until communications and data tracking were lost, the NTSB said.
"It has all the earmarks of an icing-related loss of control," said aviation safety expert Jeff Guzzetti, who used to investigate crashes for the NTSB and the Federal Aviation Administration.
The Sauk County Sheriff's Department said it received multiple reports of a downed aircraft.
No structures were damaged, and no one on the ground was injured, authorities said.
The names of the five adults and one child who died aboard the plane were not immediately released.
The NTSB will lead an investigation into the crash, which happened about 16km west of Wisconsin Dells.
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