Four Japanese troops were injured when a device suddenly exploded at a storage site for unexploded ordnance at the U.S. Air Force base on Okinawa.

Four Japanese Ground Self-Defense Force members were injured Monday after a piece of military ordnance went off at the U.S. Air Force base.

An explosion at a storage site for unexploded ordnances at a US military base on Japan’s southern island of Okinawa injured four Japanese soldiers