Efforts to locate the missing Malaysia Airlines Flight 370 drew to a close in the remote Indian Ocean on Tuesday, after a private company, hired to extend the search, admitted it had found no trace of the aircraft.

Texas-based Ocean Infinity said in a statement it had "searched and collected high-quality data from over 112,000 square kilometers of ocean floor ... overcoming both challenging conditions and terrain," but without success.

Oliver Plunkett, Ocean Infinity's chief executive, said the failure to find any sign of the plane was "extremely disappointing."

In January, the firm signed a 90-day "no find, no fee" deal with the Malaysian government to resume the hunt for the missing aircraft, a year after the official search was called off.

Big reward offered

The contract, which guaranteed Ocean Infinity $70 million (€61 million) if it found the wreckage or the black boxes, was extended twice.

Flight MH370 disappeared in March 2014 during a routine flight between Kuala Lumpur and Beijing. All 239 people on board are presumed dead. Four years on, very few details are known about what became of the plane or those on board.

Read more: MH370 report: Missing plane mystery 'unacceptable'

Several theories have emerged about MH370's fate, including a possible murder-suicide plot by pilot Zaharie Ahmad Shad, which was ruled out by investigators. 

Others include the remote hacking of the plane, a terrorist hijacking, that it ran out of fuel, or was shot down by an unknown military organization.

Investigators have had little to go on except for traces of data that the plane left on satellite and radar, and a few pieces of wreckage that have washed up on Indian Ocean islands.

The original search focused on the South China Sea, before analysis revealed the Boeing 777 had made an unexpected turn west and then south.

Australia then coordinated an official search on Malaysia's behalf that scoured 120,000 square kilometers (46,000 square miles) and cost $150 million, before it ended last January.

Read more: End of search for Malaysia Airlines plane MH370 - 'We still need answers'

'Holding out hope'

Australia's Deputy Prime Minister Michael McCormack said on Tuesday he is holding out hope that the aircraft may one day be found.

He described the four-year search as the largest in aviation history, which had tested the limits of technology as well as the capacity of experts and those working at sea.

McCormack said fresh searches may one day be launched if new technologies emerge, but doubted Australia will take part in any new search efforts "at this stage."

Malaysia's new government, meanwhile, promised to release its long-awaited report into the plane's disappearance. Transport Minister Anthony Loke said it would be published in the near future, without giving a date.

Relatives of those on board have complained that the lack of evidence, along with competing rumors and theories, has worsened their trauma.

Voice 370, a group representing the families of the victims, has called for full transparency from Malaysian investigators, after concerns that details about the plane's full cargo manifest are being covered up.

mm/rt (AFP, AP, dpa, Reuters)

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