Navy specialists were urgently trying to pick up the signal again so they can triangulate its position and go to the next step of sending an unmanned miniature submarine into the depths to try to identify plane wreckage.
Confirmation that the signals picked up by the Australian navy ship Ocean Shield belong to Flight 370's black boxes could take days, but the discovery offers "a most promising lead" yet, said Angus Houston, the head of a joint agency coordinating the multinational search. They were stronger and lasted longer than faint signals a Chinese ship reported hearing farther south in the search zone in the remote Indian Ocean.
"Clearly this is a most promising lead, and probably in the search so far, it's probably the best information that we have had," Houston said at a news conference. "We've got a visual indication on a screen and we've also got an audible signal — and the audible signal sounds to me just like an emergency locator beacon."
After a monthlong search for answers filled with dead ends, Monday's news brought fresh hope given that the two black boxes, which contain flight data and cockpit voice recordings, are the key to unraveling exactly what happened to Flight 370 and why.
In Kuala Lumpur, Malaysian Defense Minister Hishammudin Hussein told reporters that in light of the new information, "We are cautiously hopeful that there will be a positive development in the next few days, if not hours."
Little time is left to locate the devices, which have beacons that emit "pings" so they can be more easily found. The beacons' batteries last only about a month — and Tuesday marks exactly one month since the plane disappeared during a flight from Kuala Lumpur to Beijing with 239 people on board.
The Ocean Shield, which is carrying high-tech sound detectors from the U.S. Navy, picked up two separate signals late Saturday night and early Sunday morning in seas far off the west Australian coast that search crews have been crisscrossing for weeks. The first signal 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.
Still, Houston cautioned that it was too early to say the transmissions were coming from the missing jet.
"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."
The ping locator is pulled behind the ship at a depth of 3 kilometers (1.9 miles) and 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 black boxes 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."
The black boxes normally emit a frequency of 37.5 kilohertz, and the signals picked up by the Ocean Shield were both 33.3 kilohertz, he said. But the manufacturer indicated the frequency of black boxes can drift in older equipment.
The Ocean Shield was slowly canvassing a small area trying to find the signal again, though that could take another day, Matthews said.
If they pick up the signal again, the crew will launch an underwater vehicle to investigate, he said. The Bluefin 21 autonomous sub can create a sonar map of the area to chart where the debris may lie 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.
That may prove tricky because the water depth is right at the limits of the sub's capability.
"It could take some days before the information is available to establish whether these detections can be confirmed as being from MH370," Houston said. "In very deep oceanic water, nothing happens fast."
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 black box.
"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."
Meanwhile, the British ship HMS Echo, was using sophisticated sound-locating equipment to try to determine whether two separate sounds heard by a Chinese ship about 555 kilometers (345 miles) away from the Ocean Shield were related to the plane. The patrol vessel Haixun 01 detected a brief "pulse signal" on Friday and a second signal on Saturday.
The crew of the Chinese ship reportedly picked up the signals 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.
The search effort continued on the ocean surface Monday. Twelve planes and 14 ships searched 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 floating in the water.
Associated Press writers Eileen Ng in Kuala Lumpur, Malaysia, and Rohan Sullivan and Kristen Gelineau in Sydney contributed to this report.
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