By Kristen Johanson

UPPER DARBY, Pa. (CBS) — Police say they are now investigating the death of a missing teen who was found shot inside a car in North Philadelphia as a murder.

Authorities say 15-year-old Xavier Stern, of Upper Darby, had been missing for three weeks before his body was discovered Friday night inside a Lincoln Town Car that had been abandoned near Broad and York Streets.

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Police say Stern took off on November 22 after a verbal dispute with his father, but wasn’t reported missing until December 10, the night before his body was found.

Upper Darby Police Superintendent Michael Chitwood says Stern was on probation at the time of his disappearance, and had cut off his ankle bracelet before he left.

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“He was involved in an incident back in March of this year,” Chitwood says, “where he allegedly pistol-whipped another young man.”

Stern was found with a gunshot wound on the left side of his head. Authorities say they did not find any ballistic evidence, and there was no gun at the scene.

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Philadelphia police are now handling the homicide investigation.