Search teams racing against time to find the flight recorders from the missing Malaysia Airlines jet crisscrossed another patch of the Indian Ocean on Saturday, four weeks to the day after the airliner vanished.
A multinational team is desperately trying to find debris floating in the water or faint sound signals from the recorders that could lead them to the aircraft and help unravel the mystery of its fate.
Finding floating wreckage is key to narrowing the search area, as officials can then use data on currents to backtrack to where the plane hit the water, and where the flight recorders may be.
Beacons in the black boxes emit "pings" so they can be more easily found, but the batteries only last about a month.
So far, there's been no sign of the Boeing 777.
Officials have said the hunt for the wreckage is among the hardest ever undertaken, and will get much harder still if the beacons fall silent before they are found.
"Where we're at right now, four weeks since this plane disappeared, we're much, much closer," said aviation expert Geoffrey Thomas, the editor-in-chief of AirlineRatings.com. "But, frustratingly, we're still miles away from finding it. We need to find some piece of debris on the water, we need to pick up the ping."
The search will get more complicated if the signal beacons fall silent.
"What we may then do is start an enormous international effort to actually survey the Indian Ocean floor," he said. Such an effort "will take years and years to do."
The recorders could help investigators determine why Flight 370, which disappeared March 8 while en route from Kuala Lumpur to Beijing with 239 people aboard, veered so far off-course.
Two ships, the Australian navy's Ocean Shield and the British HMS Echo, carrying sophisticated equipment that can hear the recorders' pings, returned on Saturday to an area investigators hope is close to where the plane went down. They concede the area they have identified is a best guess.
Up to 13 military and civilian planes and nine other ships were also taking part in the search Saturday, the agency coordinating the search said.
Weather conditions in the area, which have regularly hampered crews trying to spot debris, were fair with some rain expected, the Joint Agency Coordination Centre said.
Because the U.S. Navy's pinger locator can pick up signals to a depth of 6,100 meters (20,000 feet), it should be able to hear the plane's data recorders even if they are in the deepest part of the search zone - about 5,800 meters (19,000 feet). But that's only if the locator gets within range of the black boxes - a tough task, given the size of the search area and the fact that the pinger locator must be dragged slowly through the water at just 1 to 5 knots (1 to 6 mph).
Officials said there was no specific information that led to the underwater devices being used for the first time on Friday, but that they were brought into the effort because there was nothing to lose.
Australian Air Chief Marshal Angus Houston, head of the joint agency coordinating the operation, acknowledged the search area was essentially a best guess, and noted the time when the plane's locator beacons would shut down was "getting pretty close."
The overall search area is a 217,000-square-kilometer (84,000-square-mile) zone in the southern Indian Ocean, about 1,700 kilometers (1,100 miles) northwest of the western Australian city of Perth.
The search area has shifted each day as investigators continue to analyze what little radar and satellite data is available while factoring in where any debris may have drifted.
Australia is coordinating the ocean search, and the investigation into the plane's disappearance is Malaysia's responsibility. Australia, the U.S., Britain and China have all agreed to be "accredited representatives" of the investigation.
Meanwhile, the Air Line Pilots Association, a union that represents 30,000 pilots in North America, said in a statement that the Malaysia Airlines tragedy should lead to higher standards of plane tracking technology being adopted by the airline industry.
Malaysia jet mystery