After weeks of searching vast swaths of ocean, investigators now have their "most promising" lead yet in finding Malaysia Airlines Flight 370.
A pinger locator in the Indian Ocean has detected signals consistent with those sent by a flight data recorder and a cockpit voice recorder, the head of the Australian agency coordinating search operations said Monday.
The sounds were heard at a depth of 4,500 meters (about 14,800 feet), retired Air Chief Marshal Angus Houston said.
"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," Houston said.
"We are encouraged that we are very close to where we need to be."
But it could take days before officials can confirm whether the signals did indeed come from the plane, which fell off the radar on March 8 with 239 people on board.
"In very deep oceanic water, nothing happens fast," Houston said. "I would ask all of you to treat this information cautiously and responsibly. ... We haven't found the aircraft yet."
At least one investigator has described the search not as finding a needle in a haystack, but rather trying to find the haystack.
"It's very exciting, very exciting," forensic audio expert Paul Ginsberg said Monday. "I think we have finally found the haystack."
And Malaysian authorities are hopeful there will be a positive development in the next few days, if not hours, acting Transportation Minister Hishammuddin Hussein told reporters Monday.
But some relatives of those on board haven't lost hope, despite Monday's news of the promising signals.
"If the plane is there, its there. We can't change it," the husband of one passenger said. "But I am still hoping for a miracle to happen."
But time could be running out in tracing the sounds. In a few hours or days, the pingers aboard the plane stop transmitting for good.
The batteries inside the beacons, which are designed to start sending signals when a plane crashes into water, last about 30 days after the devices are activated.
Monday marks the 31st day of the search.
New flight details
While searchers may be getting closer to the plane itself, a fresh mystery has emerged about what happened during the flight.
The aircraft skirted Indonesian airspace as it went off the grid and veered off course, a senior Malaysian government source told CNN on Sunday.
After reviewing radar track data from neighboring countries, officials have concluded that the plane curved north of Indonesia before turning south toward the southern Indian Ocean, the Malaysian source said.
Whoever was flying the plane, the source said, could have been trying to avoid radar detection.
Like most details in the case that's baffled investigators ever since the plane dropped off Malaysian military radar, it depends on whom you ask.
CNN law enforcement analyst Tom Fuentes cautioned against assuming a nefarious reason for steering the plane around Indonesia's airspace.
"I think the plane's being intentionally flown there, but I think it's still a mystery as to why. ... I think they would probably guess they're not avoiding anybody's radar, because there's a lot of radar in the area," he said. "I think they're avoiding getting shot down or colliding with another airplane."
CNN aviation analyst Miles O'Brien said the new route includes designated waypoints that pilots and air traffic controllers use.
"This particular route that is laid out happens to coincide with some of these named intersections," he said. "So what it shows is an experienced pilot somewhere in the mix on this."
Investigators haven't said who they think might have flown the plane off course or why.
But Hussein, Malaysia's acting transportation minister, said Monday that Indonesian military authorities told the chief of the Malaysian defense force that they had "no sighting" of the plane the night it disappeared.
The possibility that the plane was hijacked by someone who knew how to fly a commercial jet is still on the table. Authorities have also been investigating the plane's captain and co-pilot. And they haven't ruled out mechanical problems as a possible cause of the plane's diversion.
So far, no physical evidence of the plane's eventual whereabouts has been found, leaving many relatives of those on board trapped in uncertainty.
The HMS Echo, a British navy ship equipped with advanced detection gear, sailed into the area of the southern Indian Ocean on Monday morning (Sunday afternoon ET) where a Chinese crew had detected two audio signals.
The area of detection is roughly 1,600 kilometers (1,000 miles) west-northwest of Perth, according to coordinates reported by Chinese state media.
The arrival of the Echo will be critical to the search for the missing Boeing 777. It has state-of-the-art sonar and is capable of mapping the ocean floor, which is about 4,500 meters (2.8 miles) deep in the focused search area.
It should be able to help determine more confidently whether audio signals picked up on Friday and Saturday by the Chinese patrol ship Haixun 01 came from the plane.
The Chinese said the electronic pulses – detected only 2 kilometers (1.25 miles) apart – were consistent with those emitted by pingers on an aircraft's black boxes.
Sounds travel long distances underwater, Houston said, making it difficult to ascertain their sources. If detectors were near a pinger, they would pick up the signal for a more sustained period.
Houston also said that search authorities were informed Sunday that the Ocean Shield, an Australian naval vessel equipped with sophisticated listening equipment, has detected "an acoustic noise" in another area of the ocean to the north.
'Most promising lead'
The Ocean Shield, which has a high-tech pinger locator borrowed from the U.S. Navy, will continue to pursue the sound it heard. If that lead turns cold, it will move to the other detection area, a journey that will take at least a day, officials said.
On Monday morning, the Ocean Shield was "continuing investigations in its own area," Australian authorities said.
"At the moment, the most promising lead appears to be the one associated with Haixun 01," Houston said.
The pulses registered by the Chinese ship are of particular interest because they occurred in an area that fits with the latest calculation by experts of roughly where the plane is likely to have entered the water, Houston said.