PERTH, Australia -- Search crews on Wednesday combed the Indian Ocean in a frantic bid to relocate underwater sounds that may have come from the missing Malaysian jetliner's black boxes, three days after they were last detected.
The signals first heard late Saturday and early Sunday had sparked hopes of a breakthrough in the search for Flight 370, but Angus Houston, the retired Australian air chief marshal leading the search far off western Australia, said listening equipment on the Ocean Shield ship has picked up no traces of the sounds since then.
Finding the sound again is crucial to narrowing the search area so a submarine can be deployed to chart a potential debris field on the seafloor. If the autonomous sub was used now with the sparse data collected so far, covering all the potential places from which the pings might have come would take many days.
"It's literally crawling at the bottom of the ocean so it's going to take a long, long time," Houston said.
The locator beacons on the black boxes have a battery life of only about a month -- and Tuesday marked exactly one month since the plane vanished. Once the beacons blink off, locating the black boxes in such deep water would be an immensely difficult, if not impossible, task.
"There have been no further contacts with any transmission and we need to continue (searching) for several days right up to the point at which there's absolutely no doubt that the batteries will have expired," Houston said.
If, by that point, the U.S. Navy towed pinger locator has failed to pick up more signals, the sub will be deployed. If it maps out a debris field on the ocean floor, the sonar system on board will be replaced with a camera unit to photograph any wreckage.
On Wednesday, the Ocean Shield, an Australian navy supply ship, continued to draw the pinger across the northern end of a 75,427 square kilometre (29,121 square mile) search zone, the extends from 2,261 kilometres (1,405 miles) northwest of the Australian west coast city of Perth, Houston's Joint Agency Coordination Center said in a statement.
The British ship HMS Echo and China's Haixun 01 were using underwater acoustic equipment to search the southern end of the same zone. Up to 15 planes and 14 ships would join Wednesday's search for any trace of the plane, particularly floating wreckage which could lead investigators back to where the airliner crashed, the centre said. Tuesday's fine weather was forecast to be replaced by scattered showers on Wednesday.
Houston said Tuesday the two sounds heard Saturday and Sunday are consistent with the pings from an aircraft's black boxes.
But officials have cautioned it could take days to determine whether the sounds were connected to the plane that vanished March 8 on a flight from Kuala Lumpur, Malaysia, to Beijing with 239 on board.
And Houston also warned of past false leads -- such as ships detecting their own signals. Because of that, other ships are being kept away, so as not to add unwanted noise.
"We're very hopeful we will find further evidence that will confirm the aircraft is in that location," Houston said. "There's still a little bit of doubt there, but I'm a lot more optimistic than I was one week ago."
Such optimism was overshadowed by anguish at a hotel in Beijing where around 300 relatives of the flight's passengers -- most of whom were Chinese -- wait for information about the plane's fate.
One family lit candles on a heart-shaped cake to mark what would have been the 21st birthday of passenger Feng Dong, who had been working in construction in Singapore for the past year and was flying home via Kuala Lumpur. Feng's mother wept as she blew out the candles.
A family member of another passenger said staying together allowed the relatives to support one another through the ordeal. "If we go back to our homes now it will be extremely painful," said Steve Wang.
Investigators have not found any explanation yet for why the plane lost communications and veered far off its Beijing-bound course, so the black boxes containing the flight data and cockpit voice recorders are key to learning what went wrong.
The first sound picked up by the equipment on board the Ocean Shield lasted two hours and 20 minutes before it was lost, Houston said. The ship then turned around and picked up a signal again -- this time recording two distinct "pinger returns" that lasted 13 minutes. That would be consistent with transmissions from both the flight data recorder and the cockpit voice recorder.
The black boxes normally emit a frequency of 37.5 kilohertz, and the signals picked up by the Ocean Shield were both 33.3 kilohertz, U.S. Navy Capt. Mark Matthews said.
Houston said the frequency heard was considered "quite credible" by the manufacturer, and noted that the frequency from the Air France jet that crashed several years ago was 34 kilohertz. The age of the batteries and the water pressure in the deep ocean can affect the transmission level, he said.
The Ocean Shield is dragging a pinger locator at a depth of 3 kilometres (1.9 miles). It is designed to detect signals at a range of 1.8 kilometres (1.12 miles), meaning it would need to be almost on top of the recorders to detect them if they were on the ocean floor, which is about 4.5 kilometres (2.8 miles) deep.
Associated Press writers Kristen Gelineau in Sydney, Rod McGuirk in Canberra, Australia, and video journalists Isolda Morillo and Peng Peng in Beijing contributed to this report.
