A 17-year-old Stillwater boy was killed and four others injured Saturday in a knife attack on the Apple River in western Wisconsin. A 52-year-old Prior Lake, Minn., man was arrested, St. Croix County Sheriff Scott Knudson said.

"Thank goodness a witness had taken a photo of him," Knudson said. "Another witness located him at the exit of the tubing area, where he was taken into custody."

The victims and suspect were all tubing down the river around 3:45 p.m., he said. The attack happened just upstream from the Hwy. 35/64 bridge in Somerset Township, close to the Minnesota border to the north and east of Stillwater. The Apple River has long been a popular summer recreational destination for Twin Cities residents.

"We don't know yet who was connected to who, who knew each other or what precipitated it," Knudson said. "It's a tragic day."

Two victims were flown to a hospital and two were taken by ambulance. All were in critical condition. The boy who died was taken by ambulance to Lakeview Hospital in Stillwater, where he was pronounced dead.

The other victims were all in stable condition, ranging from serious to critical injuries to their torso or chest areas, according to a news release from the St. Croix County Sheriff's Office. They include two men from Luck, Wis., one 20 and the other 22 years old; a 24-year-old woman from Burnsville, Minn., and a 22-year-old man from Elk River, Minn.

The suspect was being held at the St. Croix County jail, and charges are expected to come Monday, the sheriff's office statement said.