KUALA LUMPUR, Malaysia — A police investigation may never determine the reason why the Malaysia Airlines jetliner disappeared, and search planes scouring the Indian Ocean for any sign of its wreckage aren't certain to find anything either, officials said Wednesday.
The assessment by Malaysian and Australian officials underscored the lack of knowledge authorities have about what happened on Flight 370. It also points to a scenario that becomes more likely with every passing day — that the fate of the Boeing 777 and the 239 people on board 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 up the jet just under an hour later, on the other side of the Malay Peninsula. Authorities say that 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 crew for any evidence suggesting they may have hijacked or sabotaged the plane. The backgrounds of the passengers, two-thirds of whom were Chinese, 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. "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 in the southern Indian Ocean.
A search there began just over two weeks ago, and now involves at least nine ships and nine planes.
The British government said a nuclear-powered submarine with advanced underwater search capability had arrived in the southern Indian Ocean.
The current search area is a 221,000-square kilometer (85,000-square mile) patch of sea roughly a 2½-hour flight from Perth.
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.
Flight Lieutenant Dave O'Brien, captain of an Australian P-3 Orion that arrived back after dark Wednesday at base Pearce near Perth, said it was another fruitless day of searching despite favorable weather and sea conditions.
"We didn't see anything at all of interest," he said. "So a fairly quiet day for us out there. However, we are back out tomorrow to try it all again."
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 would provide a wealth of information about the condition the plane was flying under and 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 battery-powered 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.
Malaysia has been criticized by the relatives of some Chinese passengers on board, who accuse it of not providing enough information or even lying about what it knows about the final movements of the plane. In the early days of the crisis, the Chinese government itself expressed irritation at the speed of the probe and the lack of information.
On Wednesday, China's ambassador to Malaysia sought to distance the government from the more strident criticism, perhaps concerned about any lasting damage to ties between Beijing and Kuala Lumpur.
"I wish to responsibly point out that these extreme and even somewhat irresponsible views are not representative of the overall group of Chinese relatives and even more so not representative of the Chinese government's attitude," Huang Huikang told reporters.
Scores of relatives are staying in hotels in Beijing and Kuala Lumpur, courtesy of Malaysia Airlines.
Authorities organized a closed-door briefing in Malaysia for the families with officials and experts involved in the hunt, including the chief of the Malaysian air force.
It was relayed by video conferencing technologies to the relatives in Beijing. Malaysia's civil aviation chief, Azharuddin Abdul Rahman, said officials answered all the questions raised by the relatives and that they had "a very good meeting." Several relatives interviewed after the session said officials showed them more satellite and other data, but that they were still not satisfied.
"The fact is they didn't give us any convincing information," said Steve Wang, a representative of some of the Chinese families in Beijing. "They said themselves that there are many different possibilities, but they are judging on the basis of just one of them."
Malaysian officials have on occasion given conflicting accounts and contradictory information over the last three weeks. They maintain they are doing their best in what it is an unprecedented situation, and stress they want the same thing as the families, namely to locate the plane as quickly as possible.
Perry reported from Perth, Australia. Associated Press writers Gillian Wong in Kuala Lumpur, Rod McGuirk in Canberra, Australia, and Kristen Gelineau in Sydney contributed to this report.