Japan Coast Guard's Gulfstream V aircraft flies in the search zone for debris from the missing Malaysia Airlines flight MH370 Tuesday. Pic: AP.

KUALA LUMPUR, Malaysia (AP) — Malaysia’s national police chief has warned that the investigation into what happened to Malaysia Airlines Flight 370 may take a long time and may never determine the cause of the tragedy.

Khalid Abu Bakar said Wednesday that the criminal investigation is still focused on four areas — hijacking, sabotage and personal or psychological problems of those on board the plane.

“Investigations may go on and on and on. We have to clear every little thing,” Khalid said. “At the end of the investigations, we may not even know the real cause. We may not even know the reason for this incident.”

The plane disappeared March 8 on a flight to Beijing from Kuala Lumpur with 239 on board. No trace of the plane has been found.