A Michigan man was charged Wednesday with three counts of first-degree intentional homicide in the shooting deaths of three teens at a river on the Wisconsin-Michigan border.

Authorities say Scott J. Johnson, 38, of Kingsford, Mich., went to the Menominee River in northern Wisconsin on July 31 and opened fire on a group of swimmers, killing three and injuring one.

A public defender appointed to represent Johnson did not immediately return a call for comment.

Teens from Johnson's neighborhood have said they saw him regularly at the East Kingsford Bridge, where he generally kept to himself. The bridge marks a popular swimming hold near Niagara on the Menominee River, which forms Wisconsin's border with Michigan's Upper Peninsula.

On July 31, Johnson emerged from the woods in camouflage clothes and, without a word, fired an undetermined number of shots from a military-type rifle, according to a criminal complaint.

All the shots were fired from the Wisconsin side of the river, according to Scott Celello, the undersheriff in Dickinson County, Mich. Johnson shot and killed two teens at close range and a third on the Michigan bank about 80 yards away, investigators have said.

A fourth person, a 20-year-old man, was treated at the scene for a shrapnel wound.

"They were all fleeing the scene when they were struck," Celello said.

Johnson surrendered Friday after an all-night manhunt.

Authorities haven't speculated on a motive for the murders or said whether they suspect a link between the shooting and accusations Johnson sexually assaulted a woman at the river the day before.

But Johnson's mother has said he may have "freaked" last week after he found out police wanted to speak to him, perhaps regarding the sexual assault allegations. Judy Johnson said he left the house about two hours before the shooting, unarmed and wearing a green T-shirt and dark shorts. She speculated that he had camouflage clothes and a weapon stashed somewhere.

Judy Johnson said her son was honorably discharged in 1994 without serving overseas and has been unemployed. She described him as despondent since his wife took their children and left him in 2001.

Marinette County District Attorney Brent DeBord is handling the case since the shots were fired from the Wisconsin side of the river.

But charges also could be filed in Dickinson County, Mich., because one of the three bodies was found on the Michigan bank. The district attorney there, Christopher Ninomiya, did not immediately return a message left Wednesday.