The two aircraft departed in the late evening hours on a routine training mission, from the Marine Corps Air Station Kaneohe Bay, and flew over the North Shore in the vicinity of Haleiwa.
Local residents later reported hearing two distinct explosions.
A resident on a beach reported seeing the two helicopters flying in the distance, then a fireball.

A massive search and rescue operation was undertaken, involving the United States Air Force, Coast Guard, Marines, Army MEDEVAC helicopter crews and Navy, along with the Honolulu Fire Department and Police Department.
The search and rescue operation ended on January 19.
Wreckage of the two helicopters was found in 325 feet (99 m) of water.
All four of the life rafts were also found on the surface during the 5-day search, but there was no evidence that they had ever been used by the helicopter crews.
The two aircraft departed in the late evening hours on a routine training mission, from the Marine Corps Air Station Kaneohe Bay, and flew over the North Shore in the vicinity of Haleiwa. Local residents later reported hearing two distinct explosions. A resident on a beach reported seeing the two helicopters flying in the distance, then a fireball.
A massive search and rescue operation was undertaken, involving the United States Air Force, Coast Guard, Marines, Army MEDEVAC helicopter crews and Navy, along with the Honolulu Fire Department and Police Department. The search and rescue operation ended on January 19. Wreckage of the two helicopters was found in 325 feet (99 m) of water. All four of the life rafts were also found on the surface during the 5-day search, but there was no evidence that they had ever been used by the helicopter crews.
2016 United States Marine Corps helicopter collision