Searchers hunting for the missing Malaysia Airlines jet raced toward a patch of the southern Indian Ocean on Sunday to determine whether a few brief sounds picked up by underwater equipment came from the plane's black boxes, whose battery-powered pingers are on the verge of dying out.
Ships scouring a remote stretch of water for the plane that vanished nearly a month ago detected three separate sounds over three days. A Chinese ship picked up an electronic pulsing signal on Friday and again on Saturday, and an Australian ship carrying sophisticated deep-sea acoustic equipment detected a signal in a different area on Sunday, the head of the multinational search said.
The two black boxes contain flight data and cockpit voice recordings that could solve one of the most baffling mysteries in modern aviation: who or what caused Flight 370 to veer radically off course and vanish March 8 while traveling from Kuala Lumpur, Malaysia, to Beijing with 239 people on board.
But there were questions about whether any of the sounds were the breakthrough that 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 black boxes.
"This is an important and encouraging lead, but one which I urge you to treat carefully," retired Australian Air Chief Marshal Angus Houston, who is coordinating the search, told reporters in Perth.
He warned that the sounds were "fleeting, fleeting acoustic events," not the more extended transmissions that would be expected.
"We are dealing with very deep water. We are dealing with an environment where sometimes you can get false indications," Houston said. "There are lots of noises in the ocean, and sometimes the acoustic equipment can rebound, echo if you like."
Searchers are racing against time to find the voice and data recorders. The devices emit "pings" so they can be more easily found, but the batteries last only about a month.
China's official Xinhua News Agency reported Saturday that the patrol vessel Haixun 01 detected a "pulse signal" Friday at 37.5 kilohertz — the same frequency used by the airliner's black boxes.
Houston confirmed the report and said the Haixun 01 detected a signal again on Saturday within 2 kilometers (1.4 miles) of the original signal, for 90 seconds. He said China also reported seeing floating white objects in the area.
The British navy ship HMS Echo, which is fitted with sophisticated sound-locating equipment, arrived in the area to join the search, Britain reported.
The Australian navy's Ocean Shield, which is carrying high-tech sound detectors from the U.S. Navy, will also head there, but will first investigate the sound it picked up in its current region, about 300 nautical miles (555 kilometers) away, Houston said.
Australian military aircraft are also being sent into the Haixun 01's area to investigate, he said.
In Kuala Lumpur, families of passengers aboard the missing plane attended a prayer service Sunday that also drew thousands of Malaysian supporters.
"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, president of the government coalition party that organized the two-hour session.
Two Chinese women were in tears and hugged by their caregivers after the rally. Several people wore T-shirts that read "Pray for MH370." Two-thirds of Flight 370's passengers were Chinese.
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 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.
Footage on China's state-run CCTV showed crew members poking into the water a device shaped like a large soup can attached to a pole. It was connected by cords to electronic equipment in a padded suitcase.
"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 kHz pulse was selected for underwater locator beacons because there is nothing else in the sea that naturally makes that sound, said William Waldock, an expert on search and rescue who teaches accident investigation at Embry-Riddle Aeronautical University in Arizona.
But after weeks of false alarms, officials were careful Sunday not to overplay the development.
"We are hopeful but by no means certain," Australian Prime Minister Tony Abbott said. He added: "This is the most difficult search in human history. We need to be very careful about coming to hard and fast conclusions too soon."
A senior Malaysian government official said Sunday that investigators have determined that Flight 370 skirted Indonesian airspace as it flew to the southern Indian Ocean.
The official, who spoke on condition of anonymity because he was not authorized to talk to the media, said Indonesian authorities confirmed that the plane did not show up on their military radar. The plane could have deliberately flown around Indonesian airspace to avoid detection, or may have coincidentally traveled out of radar range, he said.
Houston, the search coordinator, said there had been a correction to satellite data that investigators have been using to calculate the plane's 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, Houston said.
Up to 12 military and civilian planes and 13 ships took part in the search Sunday of three areas totaling about 216,000 square kilometers (83,400 square miles). The areas are about 2,000 kilometers (1,200 miles) northwest of the Australian coastal city of Perth.
Associated Press writers Eileen Ng in Kuala Lumpur, Malaysia, and Rohan Sullivan and Kristen Gelineau in Sydney contributed to this report.