YOKOSUKA NAVAL BASE, Japan — A box of bullets dropped from a U.S. Navy helicopter in July because of an unlucky combination of wind and vibration, according to Naval authorities.

Although all normal procedures and regulations were followed, according to Commander Naval Forces Japan, the box, containing 200 bullets for the helicopter’s machine gun, was pushed out of its bracket and fell from the helicopter gun-mount attached to the door.

The mishap occurred July 19 as the helicopter was flying from Atsugi Naval Air Facility to guard the USS Kitty Hawk as it put to sea. All but two of the bullets were found, after several days of searching, in a parking lot area in Yokohama.

No one was injured in the incident, although it caused concern among some Japanese residents and local governments, and the U.S. Navy apologized and vowed to determine what had happened.

Although no one on the helicopter saw exactly what had happened, investigators surmized that wind gusts of about 100 miles per hour, coupled with the usual vibrations of the helicopter, pushed up on the bottom of the box, disengaging it from its mount.

The Navy since has adopted additional safeguards to prevent a repeat of the accident, according to a CNFJ news release. The new safeguards include additional restraints, revised procedures for loading ammunition and moving cargo, especially over land, and added restrictions for when the helicopter doors could be open.

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