Ginowan City, Okinawa – On the morning of December 7, a group of one-year-old students were getting ready to play outside at their nursery school on the Japanese island of Okinawa when the sound of a low-flying helicopter and a loud bang rang out.

"Some children let out a cry," recalls Eriko Miyagi, a childcare worker and mother of one of the students at the Midorigaoka nursery school.

Miyagi's colleagues ran to the roof and found a cylindrical object with a red label bearing the words: "Remove before flight" and "US".

Less than a week later, on December 13, a metal window frame appeared to fall from the sky onto the grounds of a nearby elementary school.

The 7.7kg window shattered during a gym class attended by about 50 students at the Futenma No2 Elementary School. A 10-year-old boy standing less than 15 metres away suffered a minor injury.

Both incidents allegedly involved a CH-53E helicopter belonging to the US Marine Corps Air Station in Futenma, which is separated from the elementary school by only a fence.

While no children were seriously hurt, the incidents have amplified calls by the local community to halt flights over schools.

'How is this allowed to happen?'

Tomoko Miyagi has children attending both the nursery school and elementary school involved in December's incidents.

"I just couldn't understand," Miyagi tells Al Jazeera. "In what world is this allowed to happen?"

She is one of a group of parents who launched a campaign calling for a ban on military planes flying over schools.

In late December, some 600 parents and their supporters rallied in front of the city hall in Ginowan.

Earlier this month, the headmaster and members of the parent's association of Midorigaoka nursery school visited Japanese government officials in Tokyo to deliver a petition with more than 10,000 signatures. In addition to halting flights over local schools, the petition calls for a full investigation into the incident at the nursery school.

Their campaign comes as the US military - in tandem with the Japanese government - is building a US Marine Corps base in Henoko despite ongoing protests, as part of a plan to "relocate" the airbase station in Futenma to the less-populated village on Okinawa.

"We're not asking for our problems to be relocated somewhere else," says Yukiko Chinen, whose five-year-old daughter attends the nursery. "We're simply asking them to stop," she tells Al Jazeera.

According to Japan's Ministry of Defense, there were at least 25 incidents involving US military aircraft in Japan last year.

The majority took place in Okinawa, home to more than 70 percent of US military bases in Japan despite being only 0.6 percent of the country's territory.

Last year, the total number of flight "mishaps" by the US Marine Corps was the highest since 2004, according to data from the US Naval Safety Center.

So far this year, there have been at least three forced landings by US military aircraft on Okinawa.

The US military admitted that the cylindrical object that fell on the roof of the nursery school in December did belong to its CH-53 helicopter and that one of them had taken off towards to nursery school that day, but added that all cylinders on board the aircraft were accounted for.

In a statement to Al Jazeera, Gregory Cronen of the First Marine Aircraft Wing, said that "all the inventory of the type of object identified on the roof of the Midorigaoka Nursery School that were used on the 1st MAW aircraft that was operating in that area" on that day was accounted for, and that the department does not have any "further information on this object or ... its origin".

After the window incident at the Futenma elementary school, the US military vowed to try to fly around the school in the future, but within a week, helicopters could be heard overhead.

Cronen told Al Jazeera that since the incident, the First Marine Aircraft Wing had "conducted a detailed review of [its] flight patterns around Marine Corps Air Station Futenma in order to determine the safest departure and arrival patterns considering the many schools and sensitive areas located in close proximity to the base".

But on Friday, the US military admitted that a helicopter flew over the Futenma No2 Elementary school that same day, according to local media. It said it had launched an investigation into the incident.

'You'd think the war never ended'

The recent incidents are part of a layered history of occupation on the islands.

Annexed by Japan in 1879, Okinawa became the site of one of the bloodiest battles between Japan and the US during WWII, in which about 120,000 inhabitants lost their lives.

As WWII neared its end in 1945, the US military placed hundreds of thousands of Okinawans into internment camps and seized land from local villagers to build military bases throughout the island. Under the US post-war occupation, Okinawa was turned into a military hub.

Since Japan's defeat, US bases continue to occupy almost 20 percent of Okinawa's main island.

The Futenma air station, located in the densely populated city of Ginowan, occupies an area that was once home to roughly 10,000 residents. Today, a number of schools, hospitals and houses surround the base.

"Our grandparents were given no choice but to build homes and schools around the base," says Eriko Miyagi, who also grew up near the air station.

"With all these helicopters, you'd think the war never ended," Chinen tells Al Jazeera.

For those who grew up near the base and now have children at the school, they say they feel "nothing has changed."

Yoshino Oshiro, whose two children attend the nursery school, says a CH-53D helicopter crashed into one of her school's buildings when she was a university student.

"When I heard the news [in December], it immediately brought me back to that day," says Oshiro.

Chiemi Yonashiro, who attended the Futenma No2 Elementary school as a child and whose three-year-old daughter currently attends the nursery school, says it was normal to hear low-flying military planes.

"It was normal for the teacher to suspend class because of the noise from military planes," Yonashiro says.

'Children don't forget'

As parents call for a halt to flights over their children's schools, a lawsuit has also been filed by more than 22,000 residents in Kadena, Okinawa - home to the largest US military combat air wing in the Pacific - demanding a ban on nighttime and early morning flights. Since the first legal suit filed in 1982, residents have continued to speak out about the impact noise pollution from US military aircraft have had on their lives.

In February 2017, the Naha District Court granted compensation to residents claiming insomnia and hearing disorders from the noise pollution but ruled against banning flights at the Kadena airbase, saying that Japan "cannot restrict" flights at a US facility. The plaintiffs plan to appeal the court's ruling.

Shinchi Taira, one of the plaintiffs who organises weekly protests outside Kadena airbase, says that many survivors of WWII who suffer from post-traumatic stress disorder are often triggered by the sounds of military aircraft hovering overhead.

It was normal for the teacher to suspend class because of the noise from military planes. Chiemi Yonashiro, parent

The recent string of accidents, he worries, could leave a similar psychological imprint.

"Children don't forget," says Taira, who remembers watching B-52 bombers take off for Vietnam from his family's garden as an elementary school student.

In the summer of 1959, when Taira was just eight years old, an F-100 fighter jet crashed into an elementary school in central Okinawa, killing 17 people, including 11 students.

"We carry these memories throughout our lives," he says.