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.

He also said flight operations of the helicopters of the same type at Futenma had been suspended as of Wednesday afternoon.

“The safety of children should come first. It is unforgivable that it dropped in the middle of the playground,” Okinawa Gov. Takeshi Onaga told reporters as he visited the school.

The U.S. military admitted that the object was a window from a CH-53E helicopter from the Futenma base, noting that the aircraft had immediately returned to the base following the incident.

“This is a regrettable incident and we apologize for any anxiety it has caused the community,” the U.S. Marine Corps said in a media release, adding it was investigating the cause in coordination with local authorities.

The window frame, around a square meter in size, fell onto the Futenma No. 2 Elementary School playground around 10:15 a.m. The nearest student, who was attending a physical education class, was only about 5 meters away when the object came crashing down, according to a group of city assembly members who visited the site.

Shattered pieces of a window was discovered at the site, as parents, worried about their children, rushed to the school. The students in the playground at the time were second- and fourth-graders.

The incident occurred less than a week after a small cylindrical object, which local government officials believe also fell from a U.S. military aircraft, was found on the roof of a nursery school in Ginowan. The U.S. military denied that the object fell from a U.S. military aircraft, although admitting it is a component of a CH-53 helicopter.

Amid the conflicting claims of that object’s origin, the nursery school has received dozens of telephone calls and emails criticizing it for “telling a lie” or “trying to make up a story.”

“I don’t want the prefectural and central governments to accept without question what the U.S. military says,” Takehiro Kamiya, the nursery school’s principal, said Tuesday. “I want people on the mainland to think of this as their problem.”

The Japanese and U.S. 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.

The relocation is intended to remove the dangers posed by the Futenma base. But Okinawa Gov. Takeshi Onaga and many other people in the southern island prefecture, which hosts the bulk of U.S. military facilities in Japan, want the Futenma base to be removed from the prefecture altogether.

Okinawans are frustrated by the noise, crime and accidents linked to U.S. bases. Safety concerns were recently rekindled by a series of accidents involving U.S. Marines Osprey aircraft and another CH-53E helicopter that belong to the Futenma base.