(CNN) -- After weeks of searching vast swaths of ocean, the chief of the Australian agency coordinating the search for Malaysia Airlines Flight 370 said Monday that searchers have their "most promising" lead yet.



And it couldn't come soon enough -- Monday marks the 31st day of the hunt for the plane that carried 239 people.



Time could be running out in the hunt for the jetliner's cockpit recorder and flight data recorder. In a few hours or days, the pingers on those recorders could stop transmitting for good, as the batteries inside them, which are designed to start sending signals when a plane crashes into water, last about 30 days after the devices are activated.



On Sunday, a pinger locator in the Indian Ocean detected signals that were consistent with those sent by a flight data recorder and a cockpit voice recorder, retired Air Chief Marshal Angus Houston said at a news conference.



The signals were picked up 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, Australia.



The first detection lasted for more than two hours; a second lasted for about 13 minutes.



Houston called the signals the "most promising" lead, then said it was the most promising "probably in the search so far; it's probably the best information that we have had."



The sounds were heard in seawater about 4,500 meters (14,800 feet) deep, 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," he said. "We are encouraged that we are very close to where we need to be."



But he cautioned reporters to "treat this information cautiously and responsibly until such time as we can provide an unequivocal determination.



"We haven't found the aircraft yet. We need further confirmation," Houston said. "And I really stress this. It's very important."



It could take days before officials can confirm whether the signals came from the plane, which fell off radar on March 8 on its way from Kuala Lumpur, Malaysia, to Beijing.



On Monday, at least nine military aircraft, three civil aircraft and 14 ships will hunt for the plane in an area of 234,000 square kilometers (90,300 square miles), Houston said. Good weather is expected throughout the day with showers in the afternoon, he added, but rain shouldn't hurt the effort.



Need to find signal again



Capt. Mark Matthews, the U.S. Navy's salvage supervisor, is in Perth overseeing operations of the TPL-25 pinger locator and Bluefin-21 underwater vehicle on the Ocean Shield. He granted an interview Monday to CNN's Will Ripley.



He said he had cautious, measured optimism about the two signals. If they are heard again, an underwater drone could be deployed to take photos before officials could know if Flight 370 has been found. That process could take days, and possibly a week or more.



No one should jump to conclusions before the signal is confirmed to be from the aircraft, he told CNN.



Matthews praised the international team in Kuala Lumpur for a remarkable feat of engineering and mathematics that helped pinpoint the place for the Ocean Shield to begin its search, recalling how cheers erupted when the ship team reported a possible signal had been heard.



"If we can locate this wreckage and salvage the important components, it's just going to be a phenomenal event," he said. "Because I don't think in the history of aircraft searches, we've ever started with such inexact information to identify where it went in the water."



Pings detected by Chinese ship



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.



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."



The signals had a frequency that's specially designed for recorders -- a sound not occurring in nature, officials said.



Houston 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.



A reporter asked Houston on Monday if it was possible that the Chinese ship Haixon 01 heard the same thing as the Americans and Australians.



He said they were probably separate events, pointing out that the search area where they were detected is over 300 nautical miles long.



One instance "was at one extremity" he said, and "the other at the other extremity."



Finding the haystack first



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. Relatives have repeatedly heard from officials that they thought something had been found, only to have their hopes dashed when those finds have not led to the plane.



"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.



Since the plane went missing in early March, Bajc has been writing anguished notes to Wood. She speaks to him as though he's alive.



On April 5, Bajc wrote: "good evening baby ... i can hardly wait for you to come home ... i miss you so much, and worry about you constantly. please send me a sign you are ok."



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.



But why?



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.



Zeroing in



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



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."

  

CNN's David Molko, Will Ripley, Judy Kwon, Ed Payne and Mitra Mobasherat and journalist Ivy Sam also contributed to this report.