The victim's 15-year-old brother initially told emergency dispatchers about 11:30 a.m. that his brother -- identified Monday as Suhayb Jamal Hassan -- was running with a knife inside their home, fell and accidentally stabbed himself, said police detective Desiree Schroepfer, a department spokeswoman.
The 15-year-old reported that his brother was unresponsive and bleeding from a wound. The boy was pronounced dead at the scene.
Police initially described the incident as "suspicious," and after questioning, the 15-year-old admitted to investigators that he had accidentally shot his brother, Schroepfer said.
The victim was an eighth-grader at Blackhawk Middle School in Eagan, Schroepfer said.
School district officials said the school notified students of Suhayb's death Monday morning and provided additional counselors.
"Many of them knew what had happened by the time they arrived at school this morning," said Tony Taschner, spokesman for the Rosemount-Apple Valley-Eagan school district.
Police have not released the name of the 15-year-old boy. Schroepfer said the investigation was continuing.
Investigators said the boys recently had learned that their father owned a handgun and kept it in the home in the 1700 block of Silver Bell Circle.
While home alone with their 11-year-old sister, the boys found the hidden 9mm pistol.
The handgun was unloaded, but a loaded magazine was nearby.
At some point, the gun was loaded, then thought to be unloaded. The boys were chasing each other while playing "cops and robbers," Schroepfer said.
The 15-year-old pulled the trigger and unintentionally fired one round toward his brother, Schroepfer said. His brother was struck in the chest.
The sister was uninjured and did not witness the shooting, police said.