Updated

The lone gunman charged Wednesday with killing three teenagers in a river ambush planned to use the victims as bait to lure emergency responders to the scene for him to kill as well, according to a criminal complaint.

Scott J. Johnson, 38, had thought about committing a random shooting for the past four or five years and prepared by stashing weapons in the woods, according to prosecutors.

"His plan was to shoot anyone who showed up to help," Marinette County District Attorney Brent DeBord said.

He finally decided to execute his plan last week after suspecting that a woman he had recently sexually assaulted would tell her ordeal to police, the complaint said.

Johnson went to the Menominee River in northern Wisconsin on July 31 and opened fire with an Armalite 7.62 mm military-type rifle on a group of about eight swimmers, killing three and injuring one, prosecutors say.

The suspect told detectives he had been under a lot of stress and "he felt like every time he was about to obtain something, it was taken away," the complaint said.

Johnson is charged with three counts of first-degree intentional homicide. He was not charged with sexual assault, but more charges could still be filed, DeBord said at a news conference Wednesday at which he read the complaint but declined to take questions.

The homicide charges carry a maximum penalty of life in prison if he is convicted.

Johnson's public defender, Len Kachinsky, said he plans to hold his first meeting with Johnson before a court hearing Thursday.

"I think people should maintain an open mind and withhold judgment until the evidence is in," Kachinsky said.

The ambush took place at the a bridge spanning the Menominee between Wisconsin and Michigan's Upper Peninsula that is a popular swimming site.

The complaint gives the following account:

Johnson said he lured the woman near the bridge the evening before the shooting, sexually assaulted her and tried to talk her out of calling police. He decided to wait there and kill any law enforcers who arrived to investigate. When none did, he returned home.

He left his home the next day and when he returned, his mother told him police were looking for him. Expecting to go to jail and fearing the label of sex offender, he decided "he had nothing to lose and the only power he had in this life was 'to take."'

He returned to the bridge, where he counted eight teenagers. He recovered his hidden rifle and an ammunition box with .308 cartridges that he had stowed at least a year before.

He found a hill on the Wisconsin side where he could shoot. He planned to wait until the youths were back on the Michigan side and shoot them as "bait" to lure law enforcers he could then also shoot.

But he was startled when four of the teens began to climb up toward him, instead of taking the path he expected. When the teens approached, he felt trapped and jumped up, firing about 17 shots. He saw people fall as he began to reload but decided not to fire again.

Johnson spent the night in the woods and turned himself in the next morning with as officials were undertaking an intensive manhunt.

"Before exiting the woods, he disabled the firearm in a way so that law enforcement could see that the gun was disabled," DeBord said.

Two teens, Tiffany Pohlson, 17, and Anthony Spigarelli, 18, died instantly from single shots to the head. The third, Bryan Mort, 19, died of a shot to the torso. A 20-year-old man suffered superficial shrapnel wounds.

The location of the wounds suggested all four were fleeing, said Scott Celello, undersheriff for Dickinson County, Michigan.

Johnson's mother, Judy Johnson, had said her son was honorably discharged from the Army in 1994 without serving overseas and has been unemployed. She described him as despondent since his wife took left him in 2001 and took their two children with her. She said she worried he might "do something stupid."