INTERNATIONAL FALLS, Minn. (AP) — Flags flew at half-staff in Minnesota and at national parks across the country Tuesday for a Voyageurs National Park ranger who died while trying to rescue a family of three stranded by high waves on a lake.

Kevin Grossheim, 55, of Kabetogama, died Sunday while responding to a call for assistance for a father and two sons who were trapped on an island in Namakan Lake after their boat became disabled. Winds gusting over 40 mph (64 kph) had whipped up waves of 5 to 6 feet (1.5 to 1.8 meters) Sunday morning.

“One of the rangers that was out there said he has never seen conditions as wild as they were,” St. Louis County Sheriff Gordon Ramsay said.

Grossheim reached them with his boat, but it overturned after he picked them up, the National Park Service said.

“The front kind of nosedived, the boat listed, and flipped over upside-down,” Ramsay said. An initial report by the park that the ranger was towing the family’s boat proved to be inaccurate, he said.

The three family members were able to swim to shore, but Grossheim disappeared. His body was recovered that afternoon after a roughly three-hour search. A cause of death was not released.

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The remote Voyageurs, established in 1975, covers 340 square miles (880 square kilometers) of lakes, forests and streams on Minnesota’s border with Canada. Namakan Lake is one of four big lakes in the park, which also has smaller lakes. Namakan is 16 miles long and 7 miles wide at one point (16 by 11 kilometers). The park is known for boating, camping and snowmobiling; its dark skies; and abundant wildlife, including wolves, bald eagles, moose and beavers.

Grossheim had served as a ranger there for 23 years and was an experienced boat operator, familiar with navigating the lakes, the park service said.

“Kevin was much loved by all and always known to go above and beyond,” park Superintendent Bob DeGross said in a statement.

National Park Service Deputy Director Frank Lands ordered American flags lowered at national parks through sunset Wednesday in Grossheim’s honor. Gov. Tim Walz issued a similar order for state buildings in Minnesota and encouraged people, businesses and other organizations to lower their flags, too.

“Kevin Grossheim was a dedicated ranger and public servant, known for his unwavering commitment to helping others,” Walz tweeted Tuesday.