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Investigators scouring for the missing Malaysia Airlines jet will send a submarine deep into the Indian Ocean for the first time to try and figure out whether signals picked up by sound-locating equipment are from Flight 370's black boxes, taking the hunt underwater after six weeks of searching.

The crew aboard the Australian Navy vessel Ocean Shield will launch the Bluefin 21 autonomous underwater vehicle as soon as possible, said Angus Houston, the head of a joint agency coordinating the painstaking search off Australia's west coast, at a news conference just after 12 p.m. local time (12 a.m. ET).

The Bluefin 21, the Artemis autonomous underwater vehicle (AUV), is hoisted back on board the Australian Defence Vessel Ocean Shield after a successful buoyancy test in the southern Indian Ocean as part of the continuing search for the missing Malaysian Airlines flight MH370, on April 4.Australian Defence Force via Reuters

"We haven't had a single detection in six days, and I guess it's time to go underwater," Houston said.

The sophisticated sub can create a sonar map of the area to chart any debris on the seafloor.

The news comes after investigators detected a series of underwater sounds over the past two weeks that were consistent with the black boxes on the doomed aircraft, which vanished March 8 on a flight from Kuala Lumpur to Beijing with 239 passengers and crew on board.

"Analysis of the four signals has allowed the provisional definition of a reduced and manageable search area on the ocean floor. The experts have therefore determined that the Australian Defense Vessel Ocean Shield will cease searching with the towed pinger locator later today and deploy the autonomous underwater vehicle Bluefin 21 as soon as possible."

He cautioned the transition to the sub will not automatically "result in the detection of the aircraft wreckage. It may not."

Houston added that although an oil slick was located in the search area late Sunday, he was pessimistic about the chance of finding any of the floating debris.

"The chances of any floating material being recovered have greatly diminished and it will be appropriate to confer with Australia's partners to decide the way ahead later this week," Houston said.

— Daniel Arkin with The Associated Press