"We have a large number of folks that are still out in the wilderness, and we are sending additional people out to find them," Mark Van Every of the U.S. Forest Service office in Duluth said late Monday.
Rescuers airlifted the injured campers out of the Boundary Waters Canoe Area following Sunday's storms. One person was in critical condition.
Tuesday's search, using six airplanes and two helicopters, was directed at a region of 250,000 acres, about one-quarter of the huge preserve.
Van Every said most of the victims were injured by trees downed by wind clocked at 80 mph. The storms caused a swath of destruction three miles wide and 30 miles long.
"It's changed the face of the wilderness," said Roger Skraba, a longtime guide. "The trees aren't sawed off in half, they snapped off in the middle and they're laying down."
Storms also rolled from Minnesota across Lake Superior to Michigan's Upper Peninsula, where wind up to 60 mph knocked down trees and power lines.
Up to 40 percent of the 22,000 residents of Marquette were blacked out by the storm, Mayor Jack Leadbetter said.
In Arizona late Monday, four people were slightly injured by lightning as a dust storm spread across Phoenix and wind swept through Tucson, where damage included ripped-off roofs, downed power lines and uprooted trees.
Winds of 63 mph was reported at Davis-Monthan Air Force Base near Tucson.
At least two trees and two houses were struck by lightning in Tucson, the roofs of three houses were torn off and power lines fell at different points throughout the city, authorities said.
About 13,000 customers, including the state and federal prisons, were briefly left without power.