Crews search for 12 Marines missing after two helicopters collide off coast of Hawaii

(CNN) — Crews searched Friday for 12 Marines missing at sea after two CH-53 helicopters apparently collided during an overnight training flight off the coast of the Hawaiian island of Oahu, authorities said.

The U.S. Coast Guard continued the search for survivors in increasingly poor weather conditions, Marine Maj. Christian Devine said.

Marine commandant Gen. Robert B. Neller expressed hope.

“Thoughts & prayers are with our Marines & their families in Hawaii as search efforts continue. We remain Semper Fidelis,” Neller tweeted.

Earlier, searchers spotted a fire and debris field, including an empty life raft, about 2½ miles north of Haleiwa Beach, according to Coast Guard Chief Petty Officer Sara Mooers.

The aircraft were believed to have collided late Thursday off Oahu’s north shore, Mooers said.

Marine officials contacted the Coast Guard about the missing aircraft around 11 p.m. (4 a.m. Friday ET), Mooers said. A Coast Guard helicopter arrived shortly after midnight (5 a.m. ET), according to the agency.

CNN affiliate KHON in Hawaii reported that witnesses said they heard a loud boom over the water and saw a flash in the sky.

Six people were on board each of the two heavy-lift transport helicopters, according to Mooers.

The missing aircraft are assigned to Marine Heavy Helicopter Squadron 463, Marine Aircraft Group 24, 1st Marine Aircraft Wing, the Marines tweeted.

According to the unit’s website, it is stationed at Marine Corps Naval Air Station Kaneohe Bay, on the opposite side of the island from where the apparent collision occurred.

In addition to the Coast Guard helicopter, an HC-130 search plane is on scene and two cutters have also been requested, the Coast Guard said.

The U.S. Navy and Honolulu Fire Department are assisting in the operation, according to the Coast Guard.

Weather could be a factor in the search. Hawaii News Now, a CNN affiliate, reported that waves of 30 to 40 feet are expected.