Wisconsin ? A gunman suspected of opening fire on a group of young swimmers gathered along a riverbank was arrested Friday after he emerged from woods near the scene where three teenagers were slain and a fourth person was wounded.

Scott J. Johnson, 38, was dressed in camouflage when deputies confronted him following an all-night manhunt. He dropped his assault rifle as officers approached.

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 nearby 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.