From the article:
The officer attempted to detain the teenager for questioning when he threw a bottle at the sergeant's car.
Investigators said the teen then ran into an alley and the officer chased him on foot. The 15-year-old approached the officer with a "sharp object" in a threatening manner.
"He refused to follow the officer's commands to drop the object and approached the officer in an aggressive manner with the sharp object," said El Paso Police Spokesman Chris Mears. "The officer fearing for his safety fired his duty weapon striking the subject."
It isn't known how many shots were fired or where the suspect was hit. Police said he was transported to University Medical Center with non-life threatening injuries.
UPDATE (2/5/12): The boy had threatened the officer with a pair of scissors.