Searchers hunting for the missing Malaysia Airlines plane are racing to a patch of the southern Indian Ocean to determine whether a few brief sounds picked up by underwater equipment came from the jet's black boxes.
Ships scouring a remote stretch of water for the plane that vanished nearly a month ago detected three separate sounds over the course of three days.
A Chinese ship picked up an electronic pulsing signal on Friday and again on Saturday in a small part of the search zone, and an Australian ship carrying sophisticated deep-sea sound equipment picked up a signal in a different area on Sunday, the head of the multinational search said.
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But there were questions about whether any of the sounds were the breakthrough searchers are desperately seeking or just another dead end in a hunt seemingly full of them, with experts expressing doubt that the equipment aboard the Chinese ship was capable of picking up signals from the plane's two black boxes.
"This is an important and encouraging lead, but one which I urge you to treat carefully," retired Australian Air Chief Marshall Angus Houston, who is coordinating the search, told reporters in Perth.
"What we've got here are fleeting, fleeting acoustic events... That's all we've got," he said. "It's not a continuous transmission. If you get close to the device, we should be receiving it for a longer period of time than just a fleeting encounter."
None of the signals has been verified as being linked to Flight 370, which was travelling from Kuala Lumpur to Beijing when it vanished on March 8 with 239 people on board.
"We are dealing with very deep water, we are dealing with an environment where sometimes you can get false indications," Mr Houston said. "There are lots of noises in the ocean, and sometimes the acoustic equipment can rebound, echo if you like."
China's official Xinhua News Agency reported that the patrol vessel Haixun 01 detected a "pulse signal" on Friday in the southern Indian Ocean at 37.5 kilohertz - the same frequency emitted by the missing plane's black boxes.
Mr Houston confirmed the report, and said the Haixun 01 detected a signal again on Saturday within 1.4 miles of the original signal, for 90 seconds. He said China also reported seeing white objects floating in the sea in the area.
The British navy ship HMS Echo, which is fitted with sophisticated sound-locating equipment, is moving to the area where the signals were picked up and is expected to arrive early on Monday, Mr Houston said.
The Australian navy's Ocean Shield, which is carrying high-tech sound detectors from the US Navy, will also head there, but will first investigate the sound it picked up in its current region, about 300 nautical miles away, he said.
Australian air force assets are also being deployed into the Haixun 01's area to try to confirm or discount the signals' relevance to the search, Mr Houston said.
In Kuala Lumpur, families of passengers aboard the missing plane attended a prayer service on Sunday that also drew thousands of Malaysian sympathisers.
"This is not a prayer for the dead because we have not found bodies. This is a prayer for blessings and that the plane will be found," said Liow Tiong Lai, the president of the government coalition party that organised the two-hour session.
Two Chinese women were in tears and hugged by their caregivers after the rally. Many others looked somber, and several wore white T-shirts that read "Pray for MH370."
Two-thirds of the passengers aboard Flight 370 were Chinese, and a group of relatives has been in Kuala Lumpur for most of the past month to follow the investigation.
After weeks of fruitless looking, the search team is racing against time to find the sound-emitting beacons and black boxes - one a flight data recorder and the other a cockpit voice recorder - that could help unravel the mystery of the plane's fate. The beacons in the black boxes emit "pings" so they can be found more easily, but the batteries last for only about a month.
Investigators believe Flight 370 veered way off-course and came down somewhere in the southern Indian Ocean, though they have not been able to explain why it did so.
The crew of the Chinese ship reportedly picked up the signals using a hand-held sonar device called a hydrophone dangled over the side of a small runabout - something experts said was technically possible but extremely unlikely.
The equipment aboard the Ocean Shield and the HMS Echo is dragged slowly behind each ship over long distances and is considered far more sophisticated than those the Chinese crew was using.
Footage aired on China's state-run CCTV showed crew members in the small boat with a device shaped like a large soup can attached to a pole. It was hooked up by cords to electronic equipment in a padded suitcase as they poked the device into the water.
"If the Chinese have discovered this, they have found a new way of finding a needle in a haystack," said aviation expert Geoffrey Thomas, editor-in-chief of AirlineRatings.com. "Because this is amazing. And if it proves to be correct, it's an extraordinarily lucky break."
There are many clicks, buzzes and other sounds in the ocean from animals, but the 37.5 kilohertz pulse was selected for underwater locator beacons because there is nothing else in the sea that would naturally make that sound, said William Waldock, an expert on search and rescue who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Arizona.
"They picked that (frequency) so there wouldn't be false alarms from other things in the ocean," he said.
But after weeks of false alarms, officials were careful not to overplay the development.
"We are hopeful but by no means certain," Australian Prime Minister Tony Abbott said of the detection of the signals.
"This is the most difficult search in human history. We need to be very careful about coming to hard and fast conclusions too soon," Mr Abbott told reporters during a visit to Japan.
The search area has evolved as experts analysed Flight 370's limited radar and satellite data, moving from the seas off Vietnam, to the waters west of Malaysia and Indonesia, and then to several areas west of Australia.
A senior Malaysian government official said investigators have determined that the plane skirted Indonesian airspace as it flew from Peninsular Malaysia to the southern Indian Ocean.
The official, who declined to be named, said Indonesian authorities have confirmed that the plane did not show up on their military radar. The plane could have deliberately flown around Indonesian airspace to avoid radar detection, or may have coincidentally travelled out of radar range, he said.
Mr Houston, the search coordinator, conceded that his organisation first heard about the initial signal China had detected when it was reported by a Chinese journalist aboard the Haixun 01. He said that at "almost the same time" he was informed of the development by the Chinese government.
The agency was formally told about the second Chinese detection on Saturday "in absolutely the normal way," he said.
"China is sharing everything that is relevant to this search. Everything," Mr Houston said.
Still, the search agency will be adding a Chinese-speaking liaison officer "to make sure nothing falls through the cracks," he said.
Mr Houston also said there had been a correction to satellite data that investigators have been using to calculate the plane's likely flight path. As a result, starting on Monday, the southern section of the current search zone will be given higher priority than the northern part.
The signals detected by the Chinese ship were in the southern section, Mr Houston said.
Up to 12 military and civilian planes and 13 ships took part in the search on Sunday of three areas totalling about 83,400 square miles. The areas are about 1,200 miles north-west of the Australian west coast city of Perth.