Two F/A-18D Hornets with Marine All-Weather Fighter Attack Squadron 533 approach a KC-130J with Marine Aerial Refueler Transport Squadron 352 during a Special Purpose Marine Air-Ground Task Force - Crisis Response - Central Command aerial refueling exercise in an undisclosed location in Oct. 13, 2016. (Cpl. Trever Statz/Marine Corps via AP)

Three distinct signals thought to be from the flight data recorders from a Marine Corps KC-130J and F/A-18D that crashed off the coast of Japan last month have been detected, the Marine Corps confirmed Sunday.

Five crew from a KC-130J assigned to Marine Aerial Refueler Transport Squadron 152, 1st Marine Aircraft Wing, or “Sumos” squadron, and the pilot of the two-person F/A-18D Hornet assigned to Marine All Weather Fighter Attack Squadron 242, or “Bats,” were killed Dec. 6 in an accident over the Philippine Sea that has been described by Japanese defense officials as a midair collision.

The investigation into what caused the crash is expected to take months, the Marine Corps previously said.