A window of a US military transport helicopter on 13 December fell from the aircraft onto the ground of an elementary school in Japan's Okinawa Prefecture.
A student was slightly injured by small stones thrown up by the impact of the metal-framed window when it hit the playground of the school in the city of Ginowan, located just next to the US Marine Corps Air Station Futenma.
About 50 students were there at that time, reported Japan's Kyodo news agency.
The US military admitted that the object is part of a CH-53E helicopter that belongs to the Futenma base.
An investigation of the cause of this incident is underway in close coordination with local authorities, according to a media report, citing Marine officials as saying, in a statement.
"This is a regrettable incident and we apologize for any anxiety it has caused the community," the statement added.
The Japanese and US Governments have been pushing to relocate the Futenma base from a densely populated area of Ginowan to the less populated Henoko coastal district of Nago, also in Okinawa.