The US military on Monday blamed human error after a window from one of its helicopters fell onto a school sports ground in Japan, sparking anger among local residents.
The military said it had conducted a thorough investigation into last Wednesday's accident and concluded that "the incident was caused by human error".
"The window in question is designed to be removed in order to assist pilot egress in an emergency situation. The appropriate procedures for ensuring the window was secured were not correctly followed," added the military in a statement.
It again apologised for what it called a "regrettable" accident.
There were no serious injuries from the window that landed at an elementary school near the Futenma marine airbase on Okinawa island.
But the incident infuriated local residents, who are already unhappy at the base being located so close to their homes and schools.
Residents want Futenma to be closed and a replacement built elsewhere in another part of Japan or overseas, saying they can no longer live with the noise, accidents and occasional crimes committed by US service members.
"We strive to be good members of the Okinawan community and to ensure the safety of both our personnel and our community in which we live and serve," said the military statement.
The accident came just two months after an American military helicopter burst into flames after landing in an empty field in Okinawa.