Human error — not mechanical problems with the Marine Corps’ fleet of aging helicopters — caused a CH-53E Super Stallion’s window to fall off midflight and land in the sports field of an Okinawa elementary school.

The window, which is designed to be removed in emergencies, was not properly secured before the helicopter took off, 1st Marine Air Wing said in a statement Wednesday. Since the Dec. 13 incident, aircrews and maintainers have been thoroughly retrained on the procedures.

“All CH-53E aircraft have been thoroughly inspected, to include ensuring there are no mechanical or structural problems associated with the windows,” the statement says. “1st MAW will continue to inspect and conduct comprehensive maintenance on all of our aircraft in order to conduct safe flight operations. Additionally, we have conducted a review with maintenance crews of proper maintenance procedures and commander-driven briefs to all air crew on proper flight safety procedures.”

The wing called the episode “regrettable” and apologies for causing anxiety in the local community.

Meanwhile, the wing has determined that an object that fell onto the roof of a Japanese nursery school on Dec. 7 did not come from a Marine Corps aircraft, despite media reports that the object fell from a CH-53.

The object was a cover for an In-Flight Blade Inspection System, which warns pilot inflight about blade failures on Marine CH-53E and Navy MH-53E Sea Dragon helicopters, officials said.

“All of the IBIS covers on the 1st MAW aircraft operating around Midorigaoka Nursery School the morning of Dec. 7 have been accounted for,” said 1st Lt. Karoline Foote, a spokeswoman for the wing. “We have no further information on this object or the object’s origin.”

In October, a Marine CH-53E helicopter made an emergency landing outside Okinawa’s Northern Training area after catching fire midflight. The commander of III Marine Expeditionary Force suspended all CH-53 flights for six days following the incident.