A search and rescue effort is under way for the 12 missing crew members of two U.S. Marine helicopters that crashed off the coast of Hawaii.

The CH-53 transport helicopters are thought to have collided late Thursday, about four kilometers off the north coast of Oahu.

Authorities were alerted to the crash by a man on the beach who says he witnessed a large fireball after seeing the helicopters disappear.

The aircraft, which had been on a routine nighttime training mission, did not issue any distress calls.

Authorities have spotted a debris field that spans more than 11 kilometers off the coast, and have set up a safety zone in the area. None of the crew has been found.

The Coast Guard has sent a helicopter and an HC-130 Hercules airplane to assist with the rescue effort. A Navy helicopter and several other boats and planes from local authorities are also involved in the search.

Rescuers are having to deal with rough water conditions and poor visibility, which forecasters expect to last for the next several days.

Authorities have said they are hopeful, however, that survivors will be found.

“Thoughts & prayers are with our Marines & their families in Hawaii as search efforts continue,” said General Robert Neller, commandant of the Marine Corps, on Twitter.

The helicopters belonged to Marine Heavy Helicopter Squadron 463 of the service’s 1st Marine Air Wing in Hawaii.
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