GRANTSBURG, Wis. – A fierce thunderstorm swept through a Wisconsin county packed with holiday campers, toppling trees that killed an 11-year-old girl, blowing boats ashore and injuring more than 36 people, officials said Saturday.

The storm moved across Minnesota and Wisconsin on Friday, packing winds approaching 80 mph and hail as big as softballs.

In northwestern Wisconsin’s Burnett County, at least 39 people went to hospitals after the storm toppled hundreds of trees and left several thousand utility customers without power. Authorities said most of the injured were treated and released, but three were in critical condition Saturday afternoon.

A search was under way Saturday along the St. Croix River for missing canoeists, the Wisconsin Emergency Management Office said. Boats were upended and blown ashore in the area, while an airport hangar in neighboring Douglas County collapsed onto a single-engine plane.

The storm came at one of the worst times of the year for rural Burnett County: a summer holiday weekend, when the area’s lakes and rivers attract tens of thousands of visitors, said Rhonda Reynolds, a regional director for Wisconsin Emergency Management.

The rural county 90 miles northeast of Minneapolis drew an estimated 80,000 visitors for the long weekend, she said.