An autonomous submarine was to be dispatched to the bottom of the Indian Ocean Monday evening after search crews failed to detect any more signals believed to be from Malaysia Airlines Flight 370's black boxes. 

"We haven't had a single detection in six days, and I guess it's time to go underwater," said Angus Houston, the Australian official in charge of coordinating the search off Australia's west coast.

"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 said at a news conference in Perth.

But Houston warned the switch to the submarine will not automatically "result in the detection of the aircraft wreckage. It may not."

He said the submarine will take 24 hours to do each mission, including two hours to dive, 16 hours to search the bottom, then two more hours back up and four hours to download data.

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Recovering the plane's flight data and cockpit voice recorders is essential for investigators to try to figure out what happened to Flight 370, which vanished March 8. It was carrying 239 people, mostly Chinese, while en route from Kuala Lumpur, Malaysia, to Beijing.

After analyzing satellite data, officials believe the plane flew off course for an unknown reason and went down in the southern Indian Ocean. Investigators trying to determine what happened to the plane are focusing on four areas -- hijacking, sabotage and personal or psychological problems of those on board.

Two sounds heard on April 5 by the Australian ship Ocean Shield, which was towing the ping locator, were determined to be consistent with the signals emitted from the black boxes. Two more pings were detected in the same general area Tuesday, but no new ones have been picked up since then. Houston said the search using the submarine will be "a slow and painstaking process."

The sub takes six times longer to cover the same area as the ping locator, and will need about six weeks to two months to canvass the current underwater zone. The signals are also coming from about 15,000 feet below the surface, which is the deepest the sub can dive.

A visual search for debris was also planned for Monday over 18,400 square miles of ocean centered 1,400 miles northwest of the west coast city of Perth, the center said. A total of 12 planes and 15 ships would join the two searches.

The Associated Press contributed to this report.