Officer Philip Steele tells KVOA television station that the shooting happened Friday night at the family's home in Sahuarita, south of Tucson.
Steele says officers found the 3-year-old boy with a self-inflicted gunshot wound to the face after responding to an emergency call along Calle Minerva around 7:45 p.m. The boy was pronounced dead at the scene.
Steele says the parents told investigators the firearm was inadvertently misplaced for a short time. That's when the child found it.
No charges have been filed. Steele says the investigation is ongoing.