The mother of a missing 15-year-old boy who found her son shot to death in a car in North Philadelphia told police that an anonymous caller sent her to the block where she found the body, a law enforcement source said.

The mother of Xavier Stern of Upper Darby told police that she received a call last Tuesday from someone she did not know telling her to go to the area of 24th and Park Streets to find her son, the source said.

She found the teenager's body three days later in a debris-strewn car, the source said. He was wearing a ski mask and had been shot once in the head, the source said.

Stern had been missing since Nov. 22, after he got into a fight with his family and cut off his ankle monitor, which he was required to wear after being charged with hitting another teen in the head with a BB gun in March, police said.

A warrant was issued for his arrest, but Stern's father, Richard, did not report him missing until Thursday. The father told reporters at a vigil Sunday night that he had known of his son's whereabouts until the day he reported him missing.

"He was on the run, but everyone knew where he was at," Stern said.

No motive was apparent in the case, and no arrests had been made Monday.

The boy's grandmother, answering the door at the family home, declined to comment Monday evening.