KUALA LUMPUR, Malaysia — A Malaysian police investigation into the pilots of the missing Malaysian jet might turn up nothing, the force’s chief said Wednesday, while the head of the international search effort also acknowledged that an air hunt to spot wreckage on the surface of the southern Indian Ocean was not certain of success.

The statements underscored the lack of knowledge authorities have about what happened on Flight 370 and where it may have ended, and point to a scenario that becomes more likely with every passing day — that the fate of the Boeing 777 its 239 passengers and crew might remain a mystery forever.

The plane disappeared March 8 on a flight to Beijing from Kuala Lumpur after its transponders, which make the plane visible to commercial radar, were shut off. Military radar picked it up the jet just under an hour later, on the other side of the Malay peninsula. Authorities say until then its “movements were consistent with deliberate action by someone on the plane” but have not ruled out anything, including mechanical error.

Police are investigating the pilots and crews for any evidence suggesting they may have hijacked or sabotaged the plane. The backgrounds of the passengers, two-thirds of whom were from China, have been checked by local and international investigators and nothing suspicious has been found.

“Investigations may go on and on and on. We have to clear every little thing,” Inspector General Khalid Abu Bakar told reporters in Kuala Lumpur. “At the end of the investigations, we may not even know the real cause. We may not even know the reason for this incident.”

Police are also investigating the cargo and the food served on the plane to eliminate possible poisoning of passengers and crew, he said.

The search for the plane began over the Gulf of Thailand and South China Sea where the plane’s last communications were, and then shifted west to the Strait of Malacca where it was last spotted by military radar.  Experts then analyzed hourly satellite “handshakes” between the plane and a satellite and now believe it crashed somewhere into the southern Indian Ocean.

A search there began just over two weeks ago, and now involves at least nine ships and nine planes.

The current search area is a 221,000-square-kilometer (85,000-square-mile) patch of sea roughly a 2 1/2-hour flight from Perth. The focus of the search has moved several times as experts try to estimate where the plane is most likely to have landed based on assumptions on its altitude, speed and fuel. Currents in the sea are also being studied to see where any wreckage is most likely to have drifted.

Angus Houston, the head of a joint agency coordinating the multinational search effort out of Australia, said no time frame had been set for the search to end, but that a new approach would be needed if nothing showed up.

“Over time, if we don’t find anything on the surface, we’re going to have to think about what we do next, because clearly it’s vitally important for the families, it’s vitally important for the governments involved that we find this airplane,” he told Australian Broadcasting Corp. radio.

With no other data available indicating where the plane went down, spotting wreckage is key to narrowing down the search area and ultimately finding the plane’s flight data recorders, which will provide a wealth of information about the condition the plane was flying under and possibly the communications or sounds in the cockpit.

The data recorders emit a “ping” that can be detected by special equipment towed by a ship in the immediate vicinity. But the recorders stop transmitting the “pings” about 30 days after a crash. Locating the data recorders and wreckage after that is possible, but it becomes an even more daunting task.

Houston said that only once wreckage from the plane was found “we will then be able to narrowly focus the search area so that we can start to exploit the underwater technology devices that will hopefully lead to where the aircraft is on the bottom of the ocean.”