(CNN) -- After weeks of searching vast swaths of ocean, investigators now have their "most promising" lead yet in efforts to find 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, said the head of the Australian agency coordinating search operations.
The signals were picked up Sunday by the Ocean Shield, an Australian navy ship that's towing a sophisticated U.S. pinger locator through an area about 1,750 kilometers (1,100 miles) northwest of Perth. The first detection lasted for more than two hours; a second lasted for about 13 minutes.
The sounds were heard in a part of the ocean that's about 4,500 meters (about 14,800 feet) deep, retired Air Chief Marshal Angus Houston said Monday.
"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," he 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 came from the plane, which fell off 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."
"We have a promising lead, but we have yet to get confirming evidence."
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 friends and relatives of passengers said they're not putting too much stock in Monday's news.
"Until they physically locate the bulk of the plane with the black box intact and passenger bodies, I won't believe it," said Sarah Bajc, the partner of American passenger Philip Wood.
Teams are also still investigating pings detected Friday and Saturday by a Chinese ship about 600 kilometers (375 miles) southwest of the area that the Ocean Shield is searching.
But time could be running out in tracing the sounds. In a few hours or days, the pingers aboard the plane could 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, a fresh mystery has emerged about what happened during the flight.
The jet 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 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 could have been trying to avoid radar detection, the source said.
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 Hishammuddin, Malaysia's acting transportation minister, said Monday that Indonesian military authorities told 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 Ocean Shield, whose high-tech pinger locator was borrowed from the U.S. Navy, will continue to pursue the sound it heard. If that lead turns cold, it will move to another detection area, a journey that will take at least a day, officials said.
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 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.
It should be able to help determine more confidently whether audio signals picked up 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."
Houston, the head of the Australian agency coordinating the search, said sounds travel long distances underwater, making it difficult to find their sources. If detectors were near a pinger, they would pick up the signal for a more sustained period.
The signals detected by the Chinese weren't as sustained as those picked up by the Ocean Shield, and the Chinese vessel's detection gear isn't thought to be as advanced as the U.S. pinger locator. But officials say they can't discount anything at this point.
Houston said the pulses detected by the Chinese ship are particularly notable because they occurred in an area that fits with the latest expert calculation of roughly where the plane likely entered the water.
Despite the new hints that the plane may be in the ocean, some relatives of passengers are still hoping for the best.
"If the plane is there, it's there. We can't change it," the husband of one passenger said. "But I am still hoping for a miracle to happen."
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