* Search in 12th day, no wreckage found so far
* Distraught relative protests about lack of information
* Countries seen reluctant to share militarily sensitive data
* Data from pilot's flight simulator deleted
* FBI puts its technical expertise to work (Recasts with U.S. helping investigation, adds details throughout)
By Tim Hepher, A. Ananthalakshmi and Mark Hosenball
KUALA LUMPUR/WASHINGTON, March 19 (Reuters) - The FBI is helping Malaysian authorities to analyse data from a flight simulator belonging to the captain of a missing Malaysian airliner, a U.S. official said on Wednesday as investigators grasped for clues 12 days after the plane vanished.
Malaysia's police chief, Khalid Abu Bakar, said an examination of the simulator, taken from the home of pilot Zaharie Ahmad Shah, 53, showed its data log had been cleared on Feb. 3, more than a month before the airliner, carrying 239 people, disappeared on a flight from Kuala Lumpur to Beijing.
"The experts are looking at what are the logs that have been cleared," he told a news conference.
No wreckage has been found from Malaysia Airlines Flight MH370, which vanished from air traffic control screens off Malaysia's east coast at 1:21 a.m. local time on March 8 (1721 GMT March 7), less than an hour after taking off.
Malaysia has now made available to the FBI electronic data generated by both pilots of Flight MH370, including data from a hard drive attached to the captain's flight simulator, and from electronic media used by the co-pilot, Fariq Abdul Hamid, a U.S. law enforcement official told Reuters.
The official said he could not confirm that some data had been wiped from the simulator and stressed that there was no guarantee the FBI analysis would turn up any fresh clues.
U.S. investigators had become increasingly frustrated in recent days that Malaysian authorities had not asked them for more help.
The FBI has extensive experience investigating airplane crashes, including those of TWA 800 and EgyptAir 990 off the U.S. east coast in the 1990s and Pan Am Flight 103 over Lockerbie, Scotland.
In the case of EgyptAir 990, the FBI helped air safety investigators establish that the crash was caused by a suicidal co-pilot, while in the case of Pan Am 103, the agency worked with British and U.S. intelligence to build a case against the government of Libya.
In Kuala Lumpur, at the headquarters of a search operation that has so far turned up few leads, Chinese relatives' anger over sparse information on the fate of their relatives sparked chaotic scenes on Wednesday.
Malaysia's transport minister ordered an inquiry after security guards carried out the distraught mother of a passenger from a briefing room where she had protested about a lack of transparency.
"They are just saying wait for information. Wait for information. We don't know how long we have to wait," cried the woman before being whisked away from a massive media scrum.
Minister Hishammuddin Hussein said he regretted the anguish.
"Malaysia is doing everything in its power to find MH370 and hopefully bring some degree of closure for those whose family members are missing," he said in a statement.
Prospects that a 26-nation operation would lead to quick results appeared to be dwindling, however, as investigators confirmed they were focusing on the remote southern Indian Ocean after failing to find any traces of the jet further north.
"Our top priority is being given to that area," Hishammuddin told the news conference, confirming an earlier Reuters report.
Australia is leading the search in the southern part of the southern corridor, with assistance from the U.S. Navy.
It has shrunk its search field based on satellite tracking data and analysis of weather and currents, but it still covers an area of 600,000 sq km (230,000 sq miles), roughly the size of Spain and Portugal.
The unprecedented search for the Boeing 777-200ER had focused on two vast search corridors: one arcing north overland from Laos towards the Caspian Sea, the other curving south across the Indian Ocean from west of Indonesia's Sumatra island to west of Australia.
"The working assumption is that it went south, and furthermore that it went to the southern end of that corridor," said a source close to the investigation.
That view is based on the lack of any evidence from countries along the northern corridor that the plane entered their airspace, and the failure to find any trace of wreckage in searches in the upper part of the southern corridor.
Some sources involved in the investigation have voiced fears it could stall due to the reluctance of countries in the region to share militarily sensitive radar data that might shed new light on the direction the jet took.
Two people familiar with the investigation said the search had been hampered in some cases by delays over the paperwork needed to allow foreign maritime surveillance aircraft into territorial waters without a formal diplomatic request.
"These are basically spy planes; that's what they were designed for," said one source close to the investigation, explaining the hesitance of some nations to give blanket permission for other countries to scour their waters.
Hishammuddin confirmed that some assets that could be involved in the search were waiting for diplomatic clearance.
"The search for MH370 involves diplomatic, technical and logistical challenges," he told the news conference, held in a Kuala Lumpur airport hotel that has served as a temporary crisis coordination centre and a base for dozens of news organizations.
Malaysian and U.S. officials believe the aircraft was deliberately diverted perhaps thousands of miles off course, but an exhaustive background search of the passengers and crew aboard has not yielded anything that might explain why.
If the plane did indeed end up in the southern Indian Ocean, one of the remotest places on Earth and also one of the deepest seas, it increases the chance it may never be found - and investigators may never know for sure what happened on board.
Officials believe that someone with detailed knowledge of both the Boeing 777 and commercial aviation navigation switched off two vital datalinks: the ACARS system, which relays maintenance data back to the ground, and the transponder, which enables the plane to be seen by civilian radar.
U.S. agencies have looked for evidence that anyone other than the pilots knew how disable ACARS but have found nothing.
(Additional reporting by Siva Govindasamy and Niluksi Koswanage in Kuala Lumpur, Andrea Shalal and Mark Hosenball in Washington, Jane Wardell in Sydney, Peter Apps in London, Daniel Bosley in Male and Shihar Aneez in Colombo; Writing by Alex Richardson and Tim Hepher; Editing by Nick Macfie and Ross Colvin)