He faces life in prison without parole if convicted. Wisconsin does not have the death penalty.
His public defender, Len Kachinsky, said he plans meet with his client before a court hearing Thursday.
"I think people should maintain an open mind and withhold judgment until the evidence is in," Kachinsky said.
The public defender said he was not concerned that Johnson was held for nearly a week before charges were filed, as his client is unlikely to make whatever bail is set.
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 the town of Niagara on the Menominee, which forms Wisconsin's border with Michigan's Upper Peninsula.
Johnson emerged from the woods wearing camouflage clothing and, without a word, fired 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.
Marinette County District Attorney Brent DeBord is handling the case because the shots were fired from the Wisconsin side of the river. But charges also could be filed in Michigan because one of the three bodies was found on that state's side.
The Dickinson County district attorney, Christopher Ninomiya, did not immediately return a message Wednesday.
Authorities haven't speculated on a motive or said whether they suspect a link between the shooting and accusations that 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 unarmed about two hours before the shooting. She speculated that he had camouflage clothes and a weapon stashed somewhere.
Judy Johnson said her son was honorably discharged from the military in 1994 without serving overseas and has been unemployed. She described him as despondent since his wife left him in 2001 and took their children.
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