"We believe he was in the woods and near our officers, who were also in the woods all night," said Jerry Sauve, chief's deputy sheriff in Marinette County.
Nine young adults had gathered at a railroad bridge to go swimming in the Menominee River when the gunman appeared Thursday afternoon and opened fire, authorities said.
Investigators have not determined a motive. The sheriff said there was no communication between the gunman and his victims.
Johnson, who was from the nearby town of Iron Mountain, Mich., was being held in jail to await an initial court appearance. He had not been formally charged, and the jail did not know whether he had an attorney.
A judge in Michigan's Dickinson County on Friday ordered Johnson to stay away from a 24-year-old Kingsford, Mich., woman. She says he sexually assaulted her Wednesday after they went biking, then walked across the railroad bridge and into the woods in Wisconsin.
The order prohibits Johnson from following the woman, appearing at her home or work, or threatening her. Authorities did not return calls seeking comment on how the suspected assault might be related to the shootings, but the woman's father said Friday that she wasn't among the swimmers. The Associated Press does not name people alleging sexual assault unless they provide consent.
More than 100 law enforcement officers from at least 10 agencies were called in to search for the gunman. Authorities set up roadblocks and evacuated some homes in the northern Wisconsin area just across the state line from Michigan's Upper Peninsula.
Sheriff Jim Kanikula said the shooter was only 7 to 10 feet from one victim when he fired.
"There's a saying up here that this is a great place to raise kids," said Randy Van Gasse, 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."
A woman who answered the door at a Kingsford, Mich., residence and identified herself as Johnson's mother said she was too distraught to speak with a reporter.
"They're not telling me nothing," she said tearfully. "I'm going crazy here. I wish they'd tell me something."
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 of them were from Michigan.
Spigarelli's aunt, Teresa Spigarelli, remembered her nephew as "spunky, sassy." She said he was cocky even in fifth grade and handsome enough that he could have been a model.
Niagara is about 210 miles north of Milwaukee. A sign outside town calls the community a "city of scenic beauty."
Across the Menominee River is Iron Mountain and Kingsford, Mich. The cities are known for logging and tourism and have a combined population of about 15,000.
