"We will not fly any aircraft that we think is unsafe," said US Marine Corps Commander Brigadier General Paul Rock. "We will absolutely investigate this matter, and ensure that we know what happened, and to ensure that it does not happen again before we will launch another aircraft."
Rock apologized to one of Okinawa's deputy governors for the anxiety caused by the incident.
Police recovered the window and are trying to piece together what happened. They say the window weighs almost 8 kilograms and is about a meter wide.
The city's school board says the window landed about 10 meters from students who were outside for gym class. A boy was hit by something after the window fell but was uninjured.
The helicopter is deployed at a US Marine Corps air station next to the school. It can carry up to 55 people.
The school cancelled afternoon classes.
Parents expressed anger. "I'm outraged," one father said. "A school should be a safe place but now it is such a dangerous place." "It's unbelievable," another father said. "I'm worried this will happen again."
The prefecture's governor also voiced his frustration.
"The US military just continues to cause accidents," Takeshi Onaga said. "They never listen to us about improving the situation. Once again, we must remember this reality and take necessary measures."
Chief Cabinet Secretary Yoshihide Suga said, "The US military must first and foremost ensure the safety of flights. The government will continue to call on the US side to maximize its efforts to secure safety, and minimize the impact that flights have on local residents. The Japanese side will also do its utmost to prevent a recurrence."
Past accidents involving CH-53 helicopters
A series of accidents and problems involving the US military's CH-53 helicopters have occurred in the past in Okinawa.
A CH-53 went down and burst into flames on the campus of the Okinawa International University in 2004. The campus is adjacent to the US Marine Corps Futenma Air Station. 3 crewmembers were injured in the crash and university buildings and nearby residences were damaged.
Another CH-53 made an emergency landing and caught fire on grazing land in northern Okinawa, in October.
In June, another one landed at an airport in Kumejima Island during training, after a cockpit gauge alerted the crew of mechanical problems.
120 public facilities located around the Futenma base
The Futenma air station is dubbed the world's most dangerous base. Located in Okinawa's Ginowan City, which has a population of about 100,000, the base occupies the central part of the city. It takes up about one-fourth of Ginowan's total land area.
The city says there are about 120 public facilities located near the base, including schools and medical institutions. Among them are the Futenma Daini Elementary School and the Okinawa International University.
There have been 123 accidents and other problems involving US military aircraft based at the Futenma base from when Okinawa was returned to Japan in 1972 up to the end of February this year. An average of 2.7 such incidents has occurred in Okinawa every year.
US military aircraft have dropped a number of parts and objects while flying over Okinawa
Okinawa Prefecture says there have been 67 such cases from when the prefecture was returned to Japan in 1972 to December 1st this year.
In 2015, US military aircraft dropped 8 objects while flying over Okinawa. In one case, a helicopter that belonged to Futenma base dropped a part weighing more than 200 kilograms fell into the sea.
Two cases were confirmed last year. In one, a refueling plane based at the US base in Iwakuni City, Yamaguchi Prefecture dropped a rubber part after taking off from a base in Okinawa. This May, a 2-kilogram part fell off from an F-15 fighter jet stationed at Kadena Air Base while it was flying over the sea.
In November, it was found that an F-35A stealth fighter dispatched to the Kadena base was missing part of its fuselage panels.
Every time such an incident has occurred, Okinawa Prefecture and municipalities hosting US bases have lodged protests to the US military and urged it to avoid recurrences.
On December 7th, a cylindrical object with "US" written on it was found on the roof of a childcare center about 300 meters from the Futenma air station.
The US military admitted that it came from a CH-53 helicopter, but said it was unlikely the object fell during a flight. It is still unknown how the part ended up there.