A 15-year-old Minnesota boy has accidentally fatally shot dead his 13-year-old brother while playing a game of "cops and robbers" at their home on Sunday.

gathered from reports quoting police sources that the two children had stumbled upon their father's hidden unloaded 9mm pistol and a magazine with cartridges.

They were alone at home alone with their 11-year-old sister at the time of the incident.

Investigators said they somehow loaded the gun and chased each other around playing "cops and robbers" (police and thief).

The 15-year-old reportedly fired a single shot that hit his brother in the chest, killing him on the spot.

Police and paramedics responded after the teenager reported that his brother was bleeding.

He initially said his brother had fallen while ruling and stabbed himself with a knife he held.

But he later said he had accidentally shot his brother.

The shooting "appears to be a horrible accident," police say, adding that their investigations will still undergo review.