The Temple University police officer shot and killed on Saturday had tried to arrest the 18-year-old who later killed him, police said. Slain Officer Christopher Fitzgerald was patrolling the neighborhood alone at the time, which the university’s vice president of public safety said was routine, when he alerted local law enforcement over radio that he was going to stop three people wearing black masks in relation to a spate of carjackings and robberies in the area. All three fled as he tried to stop and question them but Fitzgerald was able to catch up to one of them, 18-year-old Miles Pfeffer. Security footage then captured audio of Fitzgerald ordering Pfeffer onto the ground followed by gunshots, Philadelphia Police Staff Inspector Ernest Ransom said Tuesday. Pfeffer allegedly shot Fitzgerald point-blank as he lay on the ground before carjacking a car and fleeing the scene. A Philadelphia cop, who heard Fitzgerald’s radio call about stopping three people and had headed to the scene for back-up, arrived so quickly that she was able to detain the other two people, one of whom was Pfeffer’s brother, police said. Pfeffer was arrested Sunday and charged with murder of a law enforcement officer, robbery, carjacking and weapons crimes.