Johnson, from the nearby town of Iron Mountain, Mich., had not been formally charged and it was not immediately known whether he had an attorney.
"We believe he was in the woods and near our officers who were also in the woods all night," Sauve said at a news conference.
The motive remained unclear, Sauve said. Johnson was taken to jail to await an initial court appearance that was not immediately scheduled.
More than 100 law enforcement officers from at least 10 agencies were called in to hunt for the gunman in Thursday's attack.
Authorities had set up roadblocks and evacuated some homes in the northern Wisconsin area just across the state line from Michigan's Upper Peninsula.
Marinette County Sheriff Jim Kanikula said there was no communication between the gunman and his victims. The shooter was only 7 to 10 feet away from one victim when he fired on the group of nine young people who had gathered near a railroad bridge on the Menominee River, he said.
"There's a saying up here that this is a great place to raise kids," said Randy Van Gasse, the school superintendent in nearby Norway, Mich., where 17-year-old victim Tiffany Pohlson would have been a senior in the fall.
"That's been violated and we don't know why," he said. "This is obviously a shock to everybody."
The other dead were identified as Anthony Spigarelli, 18; and Bryan Mort, 19. A fourth victim, 20-year-old Daniel Louis Gordon, was wounded. All were also from Michigan.
Niagara is about 210 miles north of Milwaukee.
--- Associated Press Writer Gretchen Ehlke contributed to this story from Milwaukee.
