By Anshuman Daga and Tim Hepher
KUALA LUMPUR, March 19 (Reuters) - Investigations into the mystery of the missing Malaysian jet appeared to be at a deadlock on Wednesday, with an exhaustive background search of the passengers and crew showing nothing untoward and no sign that the plane could be quickly found.
Eleven days have passed since Malaysia Airlines Flight MH370 went missing, and 26 nations are struggling to search for the airliner over an area roughly the size of Australia, or more than two-thirds the size of the United States.
Malaysia's top official in charge of the unprecedented operation said it was vital to reduce the scale of the task and renewed appeals for sensitive military data from its neighbours that Malaysia believes may shed light on the airliner's fate.
"All the efforts must be used to actually narrow the corridors that we have announced - I think that is the best approach to do it. Otherwise we are in the realm of speculation again," Malaysia's Acting Transport Minister Hishammuddin Hussein told reporters late on Tuesday.
The U.S. Navy said it had switched mainly to using P-8A Poseidon and P-3 Orion aircraft instead ships and helicopters.
"The maritime patrol aircraft are much more suited for this type of operation since the search field is growing," said Navy Lieutenant David Levy, who is on board the USS Blue Ridge, the U.S. Navy ship that is coordinating the search effort.
"It's just a much more efficient way to search," he said.
Flight MH370, with 239 people on board, vanished from civilian air traffic control screens off Malaysia's east coast less than an hour after taking off from Kuala Lumpur for Beijing early on March 8.
Investigators piecing together patchy data from military radar and satellites believe that someone turned off vital datalinks and turned west, re-crossing the Malay Peninsula and following a commercial route towards India.
After that, ephemeral pings picked up by one commercial satellite suggest the aircraft flew on for at least six hours, but investigators have very little idea whether it turned north or south, triggering a search expanding across two hemispheres.
Malaysian and U.S. officials believe the aircraft was deliberately diverted, talking down the possibility that an electrical fire could have prompted pilots to turn off electrics before succumbing to the fumes, which has been the subject of widespread online speculation.
U.S. government sources said intelligence agencies had extensively analysed people on the flight but came up with no connections to terrorism or possible criminal motives.
A senior U.S. official said he was "not aware of any stones left unturned".
Unless there is some kind of breakthrough, either in the form of new data or a sighting of the plane, the investigation appears to be drifting towards deadlock, sources said.
HUGE SEARCH AREA
Diplomats and safety experts said the investigation is hampered by the reluctance of many countries to share military intelligence.
Asked how important such data would be to resolving the mystery, Hishammuddin said, "It is very important. But in the case of Malaysia, we have actually put aside national security, national interest to get to where we are today."
A senior diplomat in the region said military and government leaders were studying Malaysia's request, but there was no word so far on whether any data would be exchanged.
Malaysia says it will have to buy a new radar system after revealing what it knew of the path the airliner took after turning back across its territory.
"It looks like the ball is in (others') court now and they need to decide what sort of military and other data they are willing to share with us," a Malaysian government source said.
Analysts say it will be difficult to persuade others to do the same, especially if the result would be to reveal weakness in their own defences given the numerous maritime and territorial boundary disputes going on in the region.
"Information and intelligence exchange is very sensitive in this part of the world where there is a lot of distrust and sovereign issues," said Ian Storey, a senior fellow at Singapore's Institute of Southeast Asian Studies.
"Countries are unwilling to share sensitive intelligence because if reveals their military capabilities - or lack of capabilities."
The search covers a total area of 2.24 million nautical miles (7.68 million sq km), from central Asia to the southern Indian Ocean.
Because of its size, scale of human loss and sheer uncertainty over what happened, the missing airliner looks set to establish itself as one of the most baffling air transport incidents of all time.
A breakthrough is still possible, experts say. Wreckage could be found, but the more time elapsed since the aircraft's disappearance the more it will be scattered.
"It's a mystery and it may remain a mystery," says Elizabeth Quintalla, chief air power researcher at the Royal United Services Institute in London. (Additional reporting by A. Ananthalakshmi, Siva Govindasamy, Michael Martina and Niluksi Koswanage in Kuala Lumpur, Andrea Shalal-Esa and Mark Hosenball in Washington, Jane Wardell in Sydney and Peter Apps in London; Editing by Raju Gopalakrishnan and Alex Richardson)