The aircraft went down off the coast, and the flight recorder ("black box") was not recovered. At first, the air force believed the aircraft may have been defective. The aircraft, while practicing low level flying, flew right into the water. This sort of thing has happened before, to both military and commercial aircraft, and usually as a result of pilot error. But without the black box, the investigators had to compile flight and pilot data from other sources and run flight simulations.
Right after the June 7th crash, South Korea grounded its other three F-15Ks, and asked Boeing to delay delivery of the next two aircraft, for at least a month. The South Koreans made it clear that they will hold the manufacturer responsible if the fault turns out to be with the aircraft. The F-15K is basically an upgraded version of the F-15E, and costs about $100 million each. South Korea is buying 60 of them.
Because of the findings of the investigation, production of the F-15Ks will resume, as will deliveries.
The South Korean Air Force believes it now knows why one of its new F-15K fighter-bombers crashed, on takeoff, during a night exercise. It appears that the pilots blacked out from the G forces encountered during maneuvers the pilots performed just before the crash.