The Australian air force P-3 Orion, which has been dropping sonar buoys into the water near where four earlier sounds were heard, picked up a "possible signal" that may be from a man-made source, said Angus Houston, who is coordinating the search off Australia's west coast.
"The acoustic data will require further analysis overnight," Houston said in a statement.
If confirmed, the signal would further narrow the hunt for Malaysia Airlines Flight 370, which vanished on March 8 while flying from Kuala Lumpur to Beijing with 239 people aboard.
The Australian ship Ocean Shield picked up two underwater sounds on Tuesday, and two sounds it detected Saturday were determined to be consistent with the pings emitted from a plane's flight recorders, or "black boxes."
The Australian air force has been dropping sonar buoys to maximize the sound-detectors operating in a search zone that is now the size of the city of Los Angeles.
Royal Australian Navy Commodore Peter Leavy said each buoy is dangling a hydrophone listening device about 300 meters (1,000 feet) below the surface. Each buoy transmits its data via radio back to the plane.
The underwater search zone is currently a 1,300-square-kilometer (500-square-mile) patch of the ocean floor, and narrowing the area as much as possible is crucial before an unmanned submarine can be sent to create a sonar map of a potential debris field on the seabed.
The Bluefin 21 sub takes six times longer to cover the same area as the pinger 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, U.S. Navy Capt. Mark Matthews said.
The search for floating debris on the ocean surface was narrowed Thursday to its smallest size yet — 57,900 square kilometers (22,300 square miles), or about one-quarter the size it was a few days ago. Fourteen planes and 13 ships were looking for floating debris, about 2,300 kilometers (1,400 miles) northwest of Perth.
Crews hunting for debris on the surface have already looked in the area they were crisscrossing on Thursday, but were moving in tighter patterns, now that the search zone has been narrowed to about a quarter the size it was a few days ago, Houston said.
Houston has expressed optimism about the sounds detected earlier in the week, saying on Wednesday that he was hopeful crews would find the aircraft — or what's left of it — in the "not-too-distant future."
Separately, a Malaysian government official said Thursday evening 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.
Investigators suspect the plane went down in the southern Indian Ocean based on a flight path calculated from its contacts with a satellite and analysis of its speed and fuel capacity, but the content of the flight data and cockpit voice recorders is essential to solving the mysteries of why the plane was lost.
The search for the black boxes is increasingly urgent because their locator beacons have batteries that last about a month and may fail soon.
An Australian government briefing document circulated among international agencies involved in the search on Thursday said the acoustic pingers likely would continue to transmit at decreasing strength for up to 10 more days, depending on conditions.
Once there is no hope left of the Ocean Shield hearing more sounds, the Bluefin sub will be deployed.
Complicating matters, however, is the depth of the seafloor in the search area. The pings detected earlier are emanating from 4,500 meters (14,763 feet) below the surface — which is the deepest the Bluefin can dive.
"It'll be pretty close to its operating limit. It's got a safety margin of error and if they think it's warranted, then they push it a little bit," said Stefan Williams, a professor of marine robotics at Sydney University.
The search coordination center said it was considering options in case a deeper diving sub is needed. But Williams suspects if that happens, the search will be delayed while an underwater vehicle rated to 6,000 meters (19,700 feet) is dismantled and air freighted from Europe, the U.S. or Japan.
Williams said colleagues at the Woods Hole Oceanographic Institution in Massachusetts had autonomous and remotely operated underwater vehicles that will dive to 11 kilometers (36,100 feet), although they might not be equipped for such a search.
Underwater vessels rated to 6,500 meters (21,300 feet) could search the seabed of more than 90 percent of the world's oceans, Williams said.
"There's not that much of it deeper than 6 1/2 kilometers," he said.
Williams said it was unlikely that the wreck had fallen into the narrow Diamantina trench, which is about 5,800 meters (19,000 feet) deep, since sounds emanating from that depth would probably not have been detected by the pinger locator.
Gelineau reported from Sydney. Associated Press writers Rod McGuirk in Canberra, Australia, and Eileen Ng in Kuala Lumpur, Malaysia, contributed to this report.
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