Human remains believed to be of the six Marines killed in a December mid-air collision have been recovered, Marine Corps officials announced Monday.

The remains were recovered during a salvage effort off the coast of Japan May 27 to June 7 and will be sent to Dover Air Force Base, Delaware, for identification, according to a service statement.

It remains unclear how many Marines’ remains have been recovered.

Five Marines on board a KC-130J refueler and one Marine in a F/A-18D Hornet were killed Dec. 6 when the planes collided during a training exercise off Japan.

The salvage effort also recovered the tanker’s cockpit voice recorder and digital flight recorder, according to the Marine Corps.

Those devices are being sent to Naval Air Systems Command for analysis as the investigation into the disaster continues.

Lt. Col. Kevin R. Herrmann and Maj. James M. Brophy were flying the tanker and perished in the collision.

Staff Sgt. Maximo A. Flores, Cpl. William C. Ross and Cpl. Daniel E. Baker, members of the aircrew, also died.

All were assigned to Marine Aerial Refueler Transport Squadron 152, based out of the Marine Corps Air Station Iwakuni.

Capt. Jahmar F. Resilard, a Hornet pilot with Marine All-Weather Fighter Attack Squadron 242, was in the two-seat jet at the time and died in the incident.

The lone survivor of the collision who was also in the Hornet was treated and released from the hospital, but officials have not released that Marine’s name.

“We reaffirm our deepest sympathy for all affected by this tragedy and those grieving the loss of the six Marines who perished in this accident,” the Marine Corps statement reads. “We also would like to express our sincere gratitude for the cooperation and expertise of the underwater salvage team.”