Twelve Marines are missing after a pair of helicopters collided off the northern coast of the Hawaiian island of Oahu overnight Thursday. Search and rescue efforts were still ongoing as of Friday evening, according to service spokesman Capt. Timothy Irish.

“We can confirm there are 12 Marines currently unaccounted for as a result of this incident, and officials will provide more details as they become available,” Major Christian Devine, a Marine Corps spokesman said Friday. “Our heartfelt condolences go out to the families of all those involved in the mishap, and express our deepest gratitude for the heroic, selfless actions of the civilian and military first responders.”

The Marine Corps is working with the U.S. Coast Guard in its search for survivors following the collision of the two CH-53 helicopters off of Oahu’s north coast. The CH-53 is the Marine Corps’ largest helicopter, capable of carrying more than 20 Marines each. The aircraft that collided were carrying a combined 12 people.

The aircraft were assigned to the 1st Marine Aircraft Wing. It has squadrons spread across the West Coast and Pacific.

Emergency responders arrived at the scene just after midnight local time (5 a.m. Eastern).

“Crews discovered a life raft with no one on board and visible flames on the water,” Coast Guard Petty Officer Sara Mooers told HawaiiNewsNow.

The aircraft were reportedly from the Marine Corps Air Station Kaneohe Bay. The Honolulu Fire Department and U.S. Navy assisted with the effort.

Initial reports of the crash came in around 11:40 p.m. local time with at least one witness reporting seeing a fireball. The debris had moved from about a half-mile offshore when it was discovered just after midnight to about eight miles offshore by 5:30 a.m. local time.

Rough weather has complicated the search. Waves in the area were expected to rapidly build to 30 to 40 feet by Friday morning.

CORRECTION: An earlier version of this post incorrectly noted the time difference between Hawaii-Aleutian Standard Time and the Eastern Standard Time.