A、An explosion in a naval base. B、A military attack in Honolulu. C、A killing of 12 crew members in Oahu. D、A crash of two helico

Two Marine helicopters carrying 12 crew members collided near the island of Oahu in Hawaii during a nighttime training mission, and rescuers are searching a debris field in waters on Friday, military officials said. There was no immediate word on what caused the crash or if any survivors have been found. The transport helicopters each had a crew of six from Marine Corps Base Hawaii and crashed just before midnight Thursday, officials said. No other passengers were aboard the CH-53Es, which came from the 1st Marine Aircraft Wing. A Coast Guard helicopter and C-130 airplane spotted a debris field two and a half miles offshore early Friday. The debris covers an area of two miles. The search includes aircraft from the Navy and Air Force, a Honolulu Fire Department rescue boat and Coast Guard cutters, officials said.
1. What is the news report mainly about?
2. When did the incident happen?