The U.S. military said Sunday it has categorized the fatal crash involving a fighter jet and a tanker plane off Kochi Prefecture last week as a "Class A" mishap, the most severe category on its four-level scale.

The accident occurred when an FA-18 fighter jet and a KC-130 tanker plane, both based in the nation, collided in midair early Thursday when conducting training that included refueling. A conclusion has not yet been reached on the details of the situation, according to the U.S. 3rd Marine Expeditionary Force.

Self-Defense Forces personnel rescued the two crew members of the FA-18, but one of them, Capt. Jahmar Resilard, 28, later died.