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" 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 would be analyzed, he said. If confirmed, the signal would further narrow the hunt for the Malaysia Airlines Boeing 777, which vanished March 8 while flying from Kuala Lumpur to Beijing with 239 people aboard.
The Australian ship Ocean Shield, which is towing a U.S. 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 underwater signals so they can send down a robotic submersible to look for wreckage and the flight recorders from the Malaysian jet.
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, U.S. 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."
The search for debris floating on the 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 the debris about 2,300 kilometers (1,400 miles) northwest of Perth.
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.
Investigators believe 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 recorders is essential to solving the mystery of why the plane veered so far off-course.
The search for the recorders 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 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 hearing more sounds, the Bluefin sub will be deployed.
Complicating matters, however, is the depth of the seabed in the search area. The sounds 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 shipped from Europe, the U.S. or Japan.
Williams said colleagues at the Woods Hole Oceanographic Institution in Massachusetts had autonomous and remotely operated vehicles that 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 ping locator.
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Gelineau reported from Sydney. Associated Press writers Rod McGuirk in Canberra, Australia, and Eileen Ng in Kuala Lumpur, Malaysia, contributed to this report.
