This sounds like the death of the first Confederate during the War of Northern Aggression. The soldier was hit in the chest with a spent round. The bullet did not penetrate but the trauma caused his heart to stop beating. Weird the ways we can die.
Actually if hit in the chest by an object, over the heart, in between beats the heart will stop. This has happened with basketballs and soccer balls and baseballs hitting a person, also a punch thrown at a persons chest can also have the same effect. More than likely this is what happened and if CPR had been administered immediately the kid probably would not have died.