The plane was still on the ground when an accident occurred, causing some kind of head injury, said vice president of operations at Kowalski's Markets, Mike Oase.
"It was not a crash," family spokeswoman Deb Kowalski said, "and we don't exactly know how it happened."
Kowalski, 67, was on a fishing trip with a friend and fellow pilot in Ontario province. The friend said he "heard a noise, and [Kowalski] was gone."
Kowalski and his wife took over a struggling Red Owl store in 1983 and transformed the business into a local upscale chain of stores throughout the Twin Cities area.
At the time of his death, Kowalski was still regularly involved in the day-to-day operations of the family-owned and operated store.