Japan's Defense Ministry sources have told NHK the two Maritime Self-Defense Force helicopters that crashed on Saturday had not been linked to each other on a location information sharing system.

The accident occurred during nighttime training off Torishima Island in the Izu Islands in the Pacific Ocean. It left one of the eight crew members dead and the others are missing.

The MSDF says the SH-60K patrol helicopters were equipped with a system that uses radio waves to enable wingmen to share real-time information about the target and each other's location.

The ministry sources said the system is activated in principle when two or more helicopters of this type operate together. They added that the system is designed to trigger an alarm to prevent a collision when aircraft come too close. They said that flight crews also check the locations of their wingmen visually and by radar.

MSDF officials say it is highly likely that the two helicopters collided in mid-air. They are continuing to search for the missing crewmembers while investigating why the helicopters weren't linked on the location information system.