Updated

A 15-year-old boy charged with killing his grandmother -- an off-duty Chicago police officer -- is accused of beating her with a lamp and stabbing her with a kitchen knife after she caught him skipping school, prosecutors said Sunday.

Keshawn Perkins, a ninth-grade student, was denied bond Sunday. He was charged as an adult with one count each of first-degree murder and armed robbery in the death of Hester Scott, 55, who had been a police officer for 25 years.

Scott's body was found Friday in a sewer trap behind her home on Chicago's South Side. The Cook County medical examiner's office said an autopsy Saturday showed Scott died of trauma in an assault and her death was ruled a homicide.

Scott had adopted Perkins and he lived with her, prosecutors said. The teen was represented by a public defender in court Sunday and made no comments in open court. The Cook County public defender's office was closed Sunday and was not accepting phone calls.

Prosecutors allege that Perkins had skipped school and was sleeping when his grandmother caught him. They said in court that the teen grabbed a nearby lamp and struck the woman with it "until she shut up." The teen also is accused of stabbing his grandmother with a kitchen knife, taking her purse, wrapping her body in a blanket and dumping it in the backyard.

A witness reported seeing Perkins dragging an object in the backyard and fleeing the house, authorities said.

Perkins told police where the body was, prosecutors said.