The investigation into what happened to Malaysia Airlines Flight 370 may take a long time and may never determine the cause of the tragedy, Malaysia's national police chief warned Wednesday.
Khalid Abu Bakar said the criminal investigation was 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, although searchers are now focused on a crash site in the southern Indian Ocean.
Khalid said that police had conducted more than 170 interviews with family members of the pilots and crew members. "We must be very thorough and we need all the time ... you cannot hurry us," he said.
Police are also investigating the cargo and even the food served on the plane to eliminate possible sabotage, he said.
Relatives of the passengers have been critical of the handling of the search for the plane, especially as the focus of the hunt has shifted. After experts analyzed the limited radar and satellite data from the plane, the search area was moved from the seas off Vietnam, to several areas in the Indian Ocean west of Australia, and finally to a 254,000-square-kilometer (98,000-square-mile) area roughly a 2½-hour flight from Perth.
The search resumed Wednesday, with the first of nine planes heading out to the search zone, about 1,500 kilometers (930 miles) west of Perth, the Australian Maritime Safety Authority said. Nine ships also were scouring the area.
Malaysian Prime Minister Najib Razak was expected in Perth on Wednesday, where he will meet with Australian Prime Minister Tony Abbott and visit the joint agency coordinating the multinational search effort.