A metal window frame fell off a U.S. military transport helicopter and onto the grounds of an elementary school in Ginowan, Okinawa Prefecture, on Wednesday, triggering renewed anger and concerns in the municipality where a key U.S. air base is located.

A student was slightly injured by small stones that were scattered by the impact when the object fell on the playground of the school, located just next to U.S. Marine Corps Air Station Futenma. About 60 students were there at the time.

Brig. Gen. Paul Rock, commander of Marine Corps Installations Pacific and Camp Butler, met and apologized to Okinawa Vice Gov. Moritake Tomikawa later in the day.