Seth Hammes was filming in the woods when his camcorder recorded the crack of gunshots, the 17-year-old's screams and the voice of the alleged shooter, promising help that never came.
Authorities say they might never have learned what happened to Hammes, who later died in the woods.
"But right next to him was the videotape," Monroe County Sheriff Pete Quirin said Thursday. "That's when we knew we had a homicide on our hands."
After viewing and listening to the tape, police tracked down 24-year-old Russell Schroeder, who now faces charges of reckless homicide and reckless injury. Schroeder was being held on $250,000 (Ђ207,245) bond. If convicted, he faces up to 85 years in prison, according to the AP.
Family members said Hammes and two of his friends had gone bow-hunting Saturday morning in the woods near Little Falls, about 35 miles (56 kilometers) northeast of La Crosse.
Hammes put down his bow and picked up his camcorder that afternoon.
According to the criminal complaint, he was shot in the pelvis and then the heart. His camcorder fell to the ground but caught the sound of the shots and his own screams.
The tape shows a person Quirin identified as Schroeder in a nearby field, telling Hammes he would call for help on his cell phone. He then said he couldn't find a signal but promised to go get help.
Instead, authorities say, Schroeder went to a birthday party, home to play video games and then to his job as a custodian at the Army's Fort McCoy near Sparta.
Schroeder didn't call anyone because he was scared he would get in trouble, the complaint said. He told police he thought he was shooting at a squirrel until he heard Hammes scream.
Investigators initially believed he had just died in the woods _ there was no blood to indicate foul play, and he had been shot with a .22-caliber rifle, which left only small wounds, Quirin said. Then authorities saw the camcorder tape.
Ed Hammes said his nephew loved to tinker with the family's lawn mowers and research pyrotechnics. He hung out with the La Crosse Skyrockers fireworks club, which produces the city's New Year's Eve fireworks.
"He was shy for the most part, but he'd go out of his way to help you," Ed Hammes said.