Michael Dewayne Jones, 17, told officials that he and his friends were "bored" and killed Christopher Lane for "the fun of it" while he was out for a jog in an Oklahoma neighborhood, police said.
Chancey Allen Luna, 16, was sitting in the rear seat of a car when he spotted Lane and shot him in the back, prosecutor Jason Hicks said. James Francis Edwards Jr., 15, was in the passenger seat and Jones was driving the car, Hicks said.
A 911 emergency call obtained by The Associated Press offers details of the next moments as a woman tells the operator she saw Lane fall over into a ditch as she drove by.
"He's got blood on his back," the woman says.
Later reporting information from another witness the woman said to the 911 operator: "He's turning blue. He's making a noise."
Jones was charged with using a vehicle in the discharge of a weapon and accessory to first-degree murder after the fact. Luna and Edwards were charged with first-degree murder, and will be tried as adults.
Edwards was in juvenile probation and had been in court on Friday, the day of the killing, to sign documents.
Luna's mother told KWTV in Oklahoma City her son and his friends were in a "wannabe gang." The station also reported the boys may have killed an animal the same day.
Family and friends in Australia and Oklahoma mourned Lane.
A memorial Facebook page has also been set up to honor Christopher Lane.
After the incident, Australia's former Deputy Prime Minister Tim Fischer, an advocate of gun control, has asked Australians to not visit the U.S.
"Tourists thinking of going to the USA should think twice," Fischer told the Herald Sun. "This is the bitter harvest and legacy of the policies of the NRA that even blocked background checks for people buying guns at gunshows. People should take this into account before going to the United States. I am deeply angry about this because of the callous attitude of the three teenagers (but) it's a sign of the proliferation of guns on the ground in the USA. There is a gun for almost every American."