Box said the shooting was justified because the teen was reaching into his pocket and Harrison could have assumed the teen had another weapon, since "a person with one gun may have two." Scott had previous convictions in juvenile court on misdemeanor drug charges and a pending felony case of drug possession with intent to distribute, police officials said.
Harrison, a 23-year veteran of the Del City Police Department, is scheduled to be arraigned May 14. He remains on paid administrative leave.
The chase began when police tried to stop a vehicle for a traffic violation. The car, which had three occupants including Scott, fled and then collided with a tractor-trailer in Oklahoma City.
A scuffle between Scott and Harrison ensued, then Scott took off running, said the tractor-trailer driver, David Wakefield. He said Harrison "drew his weapon and was walking very calmly toward his target, like he was at a shooting range."
Wakefield also told the AP that the teen appeared to be "holding his pants up with his left hand and running."
James Greenstone, a veteran Texas police officer and an expert in police tactics, said Monday that the accounts of the shooting suggest there was little threat posed to Harrison when he fired at Scott.
"My personal opinion is that this is just a case of bad judgment," Greenstone said. "Why would you shoot someone who is running away from you?"
Associated Press writer Katie Fretland contributed to this report from Oklahoma City.
Sean Murphy can be reached at www.twitter.com/apseanmurphy