38-year-old Scott Johnson faces three counts of first-degree intentional homicide.
Johnson is accused of opening fire on a group of swimmers at a northern Wisconsin river on Thursday. Three teens were killed, and one was injured. All four victims were from Michigan.
Johnson was arrested Friday after an all-night manhunt.
Prosecutors say Johnson's plan was to take more lives, and that he'd stashed weapons and ammo in the woods over the past year. Instead he apparently changed his mind and surrendered.
He is being charged in Wisconsin because the shots were fired from that side of the Menominee River, which divides the two states.