Hopes that a deep sea drone scouring the Indian Ocean floor might soon turn up a missing Malaysian jetliner were fading today, as the remote-controlled submarine embarked on a fifth mission with still no sign of wreckage.
Sonar footage by the US Navy owned Bluefin-21 has become the focal point of the search about 2,000km west of the Australian city of Perth.
Authorities believe Malaysia Airlines flight MH370 hit the ocean after disappearing from radars on 8 March with 239 people on board.
The search has centred on a city-sized area where a series of "pings" led authorities to believe the plane's black box may be located.
But after more than a week without a signal, and almost two weeks past the black box battery's life expectancy, authorities have now turned to the Bluefin-21.
However, after four missions to depths of about 4.5km, two of those aborted early for technical reasons, Australian search authorities said today that the drone had yet to turn up a meaningful lead.
"Bluefin-21 has searched approximately 110 square km to date.
"Data analysis from the fourth mission did not provide any contacts of interest," the Joint Agency Co-ordination Centre (JACC) said in a statement.
The centre said the Bluefin-21's search area had been reduced based on further analysis of the initial black box signals.
It said a US Navy warning that the Bluefin-21's examination may take two months was now incorrect and the drone was focusing on a "reduced and more focused underwater search area" without specifying the size.
On Monday, the search co-ordinator, retired Air Chief Marshal Angus Houston, said the air and surface search for debris would likely end in three days as the operation shifted its focus to the largely unmapped area of ocean floor.
But today, the JACC said up to 11 military aircraft and 12 ships would join in the search across 52,000sq.km of ocean.
That would suggest searchers, under pressure from the families of those on board the plane, still hold some hope of finding floating wreckage.
Australian Prime Minister Tony Abbott was quoted by the Wall Street Journal on Wednesday as saying: "We believe that (the underwater) search will be completed within a week or so. If we don't find wreckage, we stop, we regroup, we reconsider".
Asked by Reuters to clarify Mr Abbott's comments to the newspaper, his office said he was only suggesting that authorities may change the area being searched by the Bluefin-21 drone, not that the search would be called off.
Malaysia's defence minister Hishammuddin Hussein vowed that the search would continue even if there could be a pause to regroup and reconsider the best area to scour.
"The search will always continue. It's just a matter of approach," he told a news conference in Kuala Lumpur.
He said Mr Abbott remained in close contact with Malaysian Prime Minister Najib Razak and the two had spoken on Thursday to discuss the search.