The elementary school in Ginowan, Okinawa Prefecture, where the object apparently fell from a U.S. military helicopter is seen in the foreground of this photo taken from a Mainichi Shimbun aircraft. In the background is U.S. Marine Corps Air Station Futenma. (Mainichi)

GINOWAN, Okinawa -- An object resembling a window frame fell from a U.S. military helicopter onto the grounds of an elementary school here on Dec. 13, causing a stone to hit and injure a student.

The U.S. military disclosed that the object, which landed on the grounds of Futenma No. 2 Elementary School at around 10:10 a.m. on Dec. 13, fell from a CH-53 transport helicopter.

Prefectural police said that a stone flung by the force of the wind from the falling object hit a fourth-year student at the school, leaving him with a light hand injury.

The incident, which comes on the heels of another incident on Dec. 7 in which an object that was thought to be from a U.S. military aircraft fell onto a day care center in the same city, is likely to stir further protests against the U.S. military presence in the prefecture.

According to Ginowan Police Station and other sources, the object, which had an aluminum frame and measured about 90 centimeters on each side, landed near the center of the school grounds. At the time, about 50 second- and fourth-year students were on the school grounds for a P.E. class.

The Okinawa Defense Bureau of the Japanese Ministry of Defense says it was informed by the U.S. military that a window had fallen from a CH-53 transport helicopter. The school lies next to U.S. Marine Corps Air Station Futenma, separated by a fence. The Ginowan Municipal Board of Education says it has never heard of an object falling on the school.

Shortly before noon on Dec. 13, Okinawa Gov. Takeshi Onaga made a visit to the school to inspect the site. The school emailed parents to inform them of the incident. It asked parents to come and pick up their children from school early.

U.S. military helicopters return to Marine Corps Air Station Futenma, after which the fallen object was confirmed, in Ginowan, Okinawa Prefecture, on Dec. 13, 2017. (Photo courtesy of TBS Television via Ryukyu Broadcasting Corp.)

The incident is not the first involving a U.S. military aircraft near the Futenma base. In August 2004, a large U.S. transport helicopter crashed on the grounds of Okinawa International University near the base and ended up in flames. On Dec. 7 this year, a cylindrical object believed to be from a U.S. military aircraft was found at Midorigaoka day care center, several hundred meters from the Futenma base.

Dec. 13 marked one year since a U.S. military Osprey transport aircraft crash-landed in shallow waters just off the coast of Nago in Okinawa Prefecture.

In a news conference on Dec. 13, Chief Cabinet Secretary Yoshihide Suga commented, "We received a report from the U.S. that an object thought to be a door had fallen from a U.S. military helicopter." He added, "This kind of incident not only makes those at the school feel uneasy, but also causes unease to residents of Okinawa Prefecture, and is the kind of thing that must not happen." He indicated that an explanation would be provided to the Okinawa Prefectural Government and related bodies as soon as the U.S. provided details.

Defense Minister Itsunori Onodera told reporters he would like to request the suspension of flights until safety was confirmed.