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PERTH, Australia - An air and sea hunt for the missing Malaysian jet resumed Friday in the same swath of the southern Indian Ocean where an underwater sensor made the fifth detection of a signal in recent days, raising hopes that searchers are closing in on what could be a flight recorder.

This chart released by Australia's Joint Agency Coordination Center shows search area being undertaken April 11, 2014.

An Australian air force P-3 Orion, which has been dropping sonar buoys into the water near where four sounds were heard earlier, picked up a "possible signal'' on Thursday that may be from a man-made source, said Angus Houston, who is coordinating the search for Flight 370 off Australia's west coast.

The latest acoustic data needed to be analyzed, he said. If confirmed, the signal would further narrow the hunt for the plane, which vanished March 8 while flying from Kuala Lumpur to Beijing with 239 people aboard.

The Australian ship Ocean Shield, which is towing a US Navy device to detect signal beacons from a plane's flight data and cockpit voice recorders, picked up two underwater sounds Tuesday. Two sounds it detected Saturday were determined to be consistent with the pings emitted from the flight recorders, or "black boxes.''

The searchers are trying to pinpoint the location of the source of the signals so they can send down a robotic submersible to look for wreckage and the flight recorders from the Malaysian jet.

Houston's coordination center said the area to be searched for floating debris on Friday had been narrowed to 46,713 square kilometers (18,036 square miles) of ocean extending from 2,300 kilometers (1,400 miles) northwest of Perth. Up to 15 planes and 13 ships would join Friday's search.

Thursday's search of a 57,900 square kilometer (22,300 square mile) area of ocean in a similar location reported no sightings of potential wreckage, the center said.

The sonar buoys are being dropped by the Australian air force to maximize the sound-detectors operating in the search zone. Royal Australian Navy Commodore Peter Leavy said each buoy is dangling a hydrophone listening device about 300 meters (1,000 feet) below the surface and transmits its data via radio back to a search plane.

The underwater search zone is currently a 1,300-square-kilometer (500-square-mile) patch of the ocean floor _ about the size of the city of Los Angeles _ and narrowing the area as much as possible is crucial before the submersible is sent to create a sonar map of a potential debris field on the seabed.

The Bluefin 21 submersible takes six times longer to cover the same area as the ping locator being towed by the Ocean Shield and would take six weeks to two months to canvass the current underwater search zone. That's why the acoustic equipment is still being used to get a more precise location, US Navy Capt. Mark Matthews said.

Houston has expressed optimism about the sounds detected earlier in the week, saying Wednesday that he was hopeful crews would find the aircraft _ or what's left of it _ in the "not-too-distant future.''

Crews searching the surface are moving in tighter patterns, now that the zone has been narrowed to about a quarter the size it was a few days ago, Houston said.

Separately, a Malaysian government official said Thursday that investigators have concluded the pilot spoke the last words to air traffic control, "Good night, Malaysian three-seven-zero,'' and that his voice had no signs of duress. A re-examination of the last communication from the cockpit was initiated after authorities last week reversed their initial statement that the co-pilot was speaking different words.

The senior government official spoke on condition of anonymity because he wasn't authorized to speak to the media. The conclusion was first reported by CNN.