* Monday's announcement paves way for crash investigation
* Likely to be even tougher that 2009 Air France case
* Plane debris, if found, should give clues to what happened
By Siva Govindasamy and Tim Hepher
KUALA LUMPUR, March 24 (Reuters) - Confirmation that a missing Malaysian airliner crashed in the Indian Ocean opens the way for what could be one of the most costly and challenging air crash investigations in history.
Malaysian Prime Minister Najib Razak said Flight MH370 had ended far from any runway, signalling a shift of focus from a search for survivors to a mission to recover bodies and hunt for debris and black boxes.
Based on fresh analysis of satellite data by UK-based Inmarsat, Malaysia's decision to declare the aircraft lost in international waters marks a new phase in a search which has narrowed to an area southwest of Perth, Australia.
A civil investigation is likely to be carried out by Malaysia with the support of others, two people familiar with the matter said, ending two weeks of apparent legal limbo.
The launch of an official air crash investigation would give Malaysia power to coordinate and sift evidence, but it may still face critics, especially China.
Beijing has criticised Malaysia over the progress of the search and on Monday demanded that it hand over satellite data.
Inmarsat said its latest technical analysis indicated the jet had crashed in the Indian Ocean west of Perth.
Despite unconfirmed reports of possible debris, experts said it remained a mammoth task to locate wreckage without getting a closer fix on where the Boeing 777, with 239 passengers and crew on board, came down.
"If there is no tighter estimate of the location, they face the same challenges as before, looking for debris and trying to trace backwards," said Matthew Greaves, head of the Safety and Accident Investigation Centre at Britain's Cranfield University.
France's air crash investigation agency, the BEA, which has briefed Malaysia on its experience in searching for the wreckage of an Air France jet that crashed in the Atlantic in 2009, said the search area would have to be narrowed before it would be any use starting to search underwater.
It took French investigators two years to find the wreckage of the Airbus A330, long after the emergency locating beacons on its black box recorders had stopped pinging their position.
"I think this will be considerably more challenging because (Air France) AF447 crashed close to its last known position and wreckage was found within days, so that gave a much smaller area of seabed to search," said Paul Hayes, director of safety at UK-based aviation consultancy Flightglobal Ascend.
The official cost for the French-led underwater search for AF447 was 32 million euros ($44 million), but experts say it may have cost three or four times more after foreign and military costs. The current operation is seen as even more complex.
Once any debris is discovered and examined, forensic experts will try to extract as much information as they can about the aircraft's angle and condition when it entered the water.
Signs of bending or crushing can give some idea of the force of impact, while the concentration of different parts in one area might indicate whether the plane was intact or broke up at height. Even individual rivets can offer up some clues.
"Investigators are skilled at taking a huge amount out of very little," said Greaves.
Malaysian police, who have started their own criminal inquiry, are investigating suspicions that the aircraft was deliberately diverted through hijack or sabotage. But investigators have not ruled out technical problems on the 11-year old plane.
Because of the peculiar circumstances in which the jet flew on for hours after changing course shortly after leaving Kuala Lumpur for Beijing on March 8, some evidence may be missing.
Cockpit voice recorders keep two hours of tape before overwriting. Malaysian investigators believe the jet turned off course about 40 minutes into the flight but flew on for hours after that. However, flight data recorders can shed some light on cockpit behaviour such as positions of pilot seats and doors.
"It will be a long time before they give up and if there is any chance of recovering wreckage they will get to the bottom of what happened, but they may never get to the bottom of why it happened," Greaves said.
Malaysia's department of civil aviation will take the lead in a civil investigation to be carried out under United Nations rules, according to two people familiar with the matter.
But its relative lack of experience means it is likely to rely on foreign agencies, with some experts predicting a key role for Australia which is coordinating search efforts.
The United States will be accredited automatically as the country where the Boeing 777 was designed and made, as will Britain which supplied the Rolls-Royce engines. China is expected to participate because of the 152 Chinese passengers. (Editing by Robin Pomeroy)