Anthony Mendoza was arrested Friday and charged with first-degree murder, according to the Cook County state's attorney's office.
The teen was shot in the chest about 11:35 a.m. in the 3500 block of South Parnell, according to Chicago Police and the Cook County medical examiner’s office.
A police source said the shooting was initially thought to be accidental, happening while the boys, along with Bandemer's cousin, were playing with the gun.
Bandemer, who lived in the 900 block of West 36th Street, just a few blocks from where he was was shot, was pronounced dead at the scene, authorities said.
An autopsy Saturday found he died from a shotgun wound to the chest, and his death was ruled a homicide, the medical examiner’s office said.
But Mendoza allegedly told the cousin not to talk to police about what happened, and while he was being taken in for questioning, allegedly admitted to the crime in a video post on Snapchat, sources said.
Judge Peggy Chiampas ordered Mendoza held on a $250,000 bond Sunday, and if he does post bond, no guns are to be allowed in his home. He is next scheduled to appear in court June 24.
--Chicago Sun-Times Wire