The U.S. military has ended search operations for five missing Marines involved in an accident off the coast of Japan five days ago, declaring all of them dead.

Two Marines were rescued from the ocean after an F/A 18 Hornet fighter jet and a KC-130 refueling plane collided at 2 a.m. local time Thursday. One of the Marines was subsequently declared dead. The latest news brings the death toll from the accident to six.

“Every possible effort was made to recover our crew and I hope the families of these selfless Americans will find comfort in the incredible efforts made by U.S., Japanese, and Australian forces during the search,” Lt. Gen. Eric Smith, commander of the III Marine Expeditionary Force, said in a statement.

The U.S. Navy’s 7th Fleet, Japan’s Maritime Self-Defense Force and its coast guard were among the forces that rushed to the scene of the crash off the Japanese coast.

Next of kin have been informed, and the identities of the Marines will be revealed 24 hours after their families are told.

The incident has heightened concerns about air accidents involving the U.S. military in Japan, where some 54,000 troops are stationed.

It has also added to a growing list of accidents involving U.S. military planes all over the world, raising concerns that budget cuts, 17 years of war in Afghanistan, Iraq and elsewhere, and the cost of maintaining a fleet of aging aircraft have fatally undermined safety.

The Marine Corps said the circumstances of this latest “mishap” remain under investigation.

“The aircraft were conducting regularly scheduled training. It is not confirmed that aerial refueling was ongoing when the mishap occurred,” it said in a statement. “The Marine Corps rigorously investigates all aviation mishaps to identify the causes, learn from them, and mitigate future incidents.”

The two Marines from the Hornet fighter were found in the ocean, but one had spent more than 10 hours in the water and was later declared dead. He was named last week as Capt. Jahmar F. Resilard, 28, a pilot with the Marine All Weather Fighter Attack Squadron 242, known as the Bats, stationed at Marine Corps Air Station Iwakuni in southern Japan.

Resilard was from Miramar, Fla. His decorations include the Navy and Marine Corps Achievement Medal, Global War on Terrorism Service Medal and National Defense Service Medal, the Marines said last week.

“The Bats are deeply saddened by the loss of Captain Jahmar Resilard,” Lt. Col. James Compton, commanding officer of his squadron, said Friday. “He was an effective and dedicated leader who cared for his Marines and fellow fighter pilots with passion. His warm and charismatic nature bound us together, and we will miss him terribly.”

The KC-130 Hercules was assigned to Marine Aerial Refueler Transport Squadron 152 (the Sumos), 1st Marine Aircraft Wing.

“All of us in the Sumo family are extremely saddened following the announcement of the conclusion of search and rescue operations,” said Lt. Col. Mitchell T. Maury, commanding officer of the squadron. “We know this difficult decision was made after all resources were exhausted in the vigorous search for our Marines. Our thoughts are heavy and our prayers are with all family and friends of all five aircrew.”

The planes took off from Iwakuni, one of the biggest U.S. air bases in East Asia. The base is near the southern tip of Japan’s main island of Honshu, about 25 miles from the city of Hiroshima. It hosts around 15,000 personnel, with U.S. Marines alongside units of Japan’s Maritime Self-Defense Force.