The focus of the search changed repeatedly since contact was lost with the plane between Malaysia and Vietnam. It began in the South China Sea, then shifted toward the Strait of Malacca to the west, where Malaysian officials eventually confirmed that military radar had detected the plane.
An analysis of satellite data indicated the plane veered far off course for a still-unknown reason, heading to the southern Indian Ocean, where officials say it went down at sea. They later shifted the search area closer to the west coast of Australia.
"We are cautiously hopeful that there will be a positive development in the next few days, if not hours," Malaysian Defense Minister Hishammuddin Hussein said in the capital of Kuala Lumpur.
But Angus Houston, the retired Australian air chief marshal who heads the search operation, added: "We haven't found the aircraft yet."
The Ocean Shield, an Australian ship towing sophisticated U.S. Navy listening equipment, detected two distinct, long-lasting sounds underwater that are consistent with the pings from an aircraft's "black boxes" the flight data and cockpit voice recorders, Houston said.
Navy specialists were urgently trying to pick up the signal detected Sunday by the Ocean Shield so they can triangulate its position and go to the next step of sending an unmanned miniature submarine into the depths to look for any plane wreckage.
Geoff Dell, discipline leader of accident investigation at Central Queensland University in Australia, said it would be "coincidental in the extreme" for the sounds to have come from anything other than an aircraft's flight recorder.
"If they have a got a legitimate signal, and it's not from one of the other vessels or something, you would have to say they are within a bull's roar," he said. "There's still a chance that it's a spurious signal that's coming from somewhere else and they are chasing a ghost, but it certainly is encouraging that they've found something to suggest they are in the right spot."
And in "very deep oceanic water," Houston said, "nothing happens fast."
"Clearly, this is a most promising lead," he said in Perth. "And probably in the search so far, it's probably the best information that we have had."
Houston said the signals picked up by the Ocean Shield were stronger and lasted longer than faint signals a Chinese ship reported hearing about 555 kilometers (345 miles) south in the remote search zone off Australia's west coast.
The British ship HMS Echo was using sophisticated sound-locating equipment to determine whether two separate sounds heard by the Chinese patrol vessel Haixun 01 were related to Flight 370. The Haixun detected a brief "pulse signal" on Friday and a second signal Saturday.
The Chinese reportedly were using a sonar device called a hydrophone dangled over the side of a small boat something experts said was technically possible but extremely unlikely. The equipment aboard the British and Australian ships is dragged slowly behind each vessel over long distances and is considered far more sophisticated.
Little time is left to locate the flight recorders, whose locator beacons have a battery life of about a month. Tuesday marks exactly one month since the Malaysia Airlines plane disappeared.
China's official Xinhua News Agency reported late Saturday that the signal detected by the Haixun crew was pulsing at 37.5 kilohertz the same frequency emitted by flight data recorders. Malaysia's civil aviation chief, Azharuddin Abdul Rahman, confirmed the frequency emitted by Flight 370's black boxes was 37.5 kilohertz.
The Ocean Shield picked up its signals late Saturday night and early Sunday morning.
The first lasted two hours and 20 minutes before it was lost. The ship then turned around and picked up a signal again this time recording two distinct "pinger returns" that lasted 13 minutes, Houston said.
"Significantly, this would be consistent with transmissions from both the flight data recorder and the cockpit voice recorder," Houston said.
The frequency used by aircraft flight recorders was chosen because no other devices use it, and because nothing in the natural world mimics it, said William Waldock, a search-and-rescue expert who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.
"They picked that so there wouldn't be false alarms from other things in the ocean," he said.
But these signals are being detected by computer sweeps, and "not so much a guy with headphones on listening to pings," said U.S. Navy spokesman Chris Johnson. So until the signals are fully analyzed, it's too early to say what they are, he said.
"We'll hear lots of signals at different frequencies," he said. "Marine mammals. Our own ship systems. Scientific equipment, fishing equipment, things like that. And then of course there are lots of ships operating in the area that are all radiating certain signals into the ocean."-
The Ocean Shield is dragging a ping locator at a depth of 3 kilometers (1.9 miles). It is designed to detect signals at a range of 1.8 kilometers (1.12 miles), meaning it would need to be almost on top of the recorders to detect them if they were on the ocean floor, which is about 4.5 kilometers (2.8 miles) deep.
"It's like playing hot and cold when you're searching for something and someone's telling you you're getting warmer and warmer and warmer," U.S. Navy Capt. Mark Matthews said. "When you're right on top of it, you get a good return."
While Matthews said the signals picked up by the Ocean Shield were both 33.3 kilohertz, the manufacturer indicated the frequency can drift in older equipment.
If they pick up the signal again, the crew will launch an underwater vehicle to investigate, Matthews said. The Bluefin 21 autonomous sub can create a sonar map of the area to chart any debris on the sea floor. If it maps out a debris field, the crew will replace the sonar system with a camera unit to photograph any wreckage.
The water depth there is right at the limits of the sub's capability.
Meanwhile, the search effort continued on the surface.
Twelve planes and 14 ships scoured three designated zones, one of which overlaps with the Ocean Shield's underwater search. All of the previous surface searches have found only fishing equipment or other sea trash, something that gave Houston pause.
"I would want more confirmation before we say this is it," he said. "Without wreckage, we can't say it's definitely here. We've got to go down and have a look."
Breed reported from Raleigh, N.C. Associated Press writers Eileen Ng in Kuala Lumpur, Malaysia, and Rohan Sullivan and Kristen Gelineau in Sydney contributed to this report.