UPDATE: Hamilton was officially charged Monday. DETAILS HERE.

A 15-year-old boy surrendered to Philadelphia Police in the robbery and shooting death of a man who police say was targeted because he appeared old, police told NBC10. The boy was the third suspect sought in the deadly encounter.

Tyfine Hamilton's father helped negotiate the arrest, sources said. He is now being interviewed by homicide detectives at Philadelphia Police Headquarters in Center City.

Hamilton was wanted for the murder of 51-year-old James Patrick Stuhlman. Last Thursday, Stuhlman was walking his dog, Molly, along the 6400 block of Woodcrest Avenue in the Overbrook section of the city when he was approached by three teenage boys, police said.

Homicide investigators said the teens planned to rob the man after playing a game of basketball. They chose him, investigators said, because he looked old and his dog was "weak." Stuhlman pleaded for his life before he was shot once in the chest, police said. He died on the street with his dog at his side.

Hamilton's alleged co-conspirator, 15-year-old Brandon Smith, was arrested Thursday and charged with murder after police spotted him in Overbook, police said. A 14-year-old was also arrested and charged with robbery, as well.

Stuhlman was shot over nothing, police said, as not one item was taken from him.

Hamilton has not yet been charged in the case.