PERTH, Australia (AP) -- Two ships with sophisticated equipment for searching underwater zeroed in Friday on a remote stretch of the Indian Ocean in a desperate hunt for the missing Malaysia Airlines jet's black boxes, whose batteries will soon run out.
An arduous weeks-long hunt has not turned up a single piece of wreckage, which could have led the searchers to the plane and eventually to its black boxes containing key information about the flight. But the searchers have apparently decided to make a direct attempt to find the devices, whose batteries last about a month.
Two ships with equipment that can hear the black boxes' pings were slowly making their way along a 240-kilometer (150-mile) route that investigators are hoping may be close to the spot Flight 370 entered the water after it vanished March 8 on a flight from Kuala Lumpur, Malaysia, to Beijing.
But the head of the joint agency coordinating the search acknowledged that the search area was essentially just a best guess - and noted that time was running out for search crews to find the coveted data recorders.
"The locater beacon will last about a month before it ceases its transmissions - so we're now getting pretty close to the time when it might expire," Angus Houston said.
The Australian navy ship Ocean Shield, which is dragging a towed pinger locator from the U.S. Navy, and the British navy's HMS Echo, which has underwater search gear on board, were looking for the black boxes in an area that investigators' settled on after analyzing hourly satellite pings the aircraft gave off after it disappeared. That information, combined with data on the estimated speed and performance of the aircraft, led them to that specific stretch of ocean, Houston said.
"The area of highest probability as to where the aircraft might have entered the water is the area where the underwater search will commence," he said. "It's on the basis of data that only arrived very recently and it's the best data that is available."
Because the U.S. Navy's pinger locator can pick up black box signals up to a depth of 6,100 meters (20,000 feet), it should be able to hear the devices even if they are lying in the deepest part of the search zone - about 5,800 meters (19,000 feet) below the surface. 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 the pinger locator must be dragged slowly through the water at just 1 to 5 knots, or 1 to 6 miles per hour.
Finding floating wreckage is key to narrowing the search area, as officials can then use data on ocean currents to try and backtrack to the spot where the Boeing 777 hit the water - and where the black boxes may be. The devices would provide crucial information about what condition the plane was flying under and any communications or sounds in the cockpit.
But with no wreckage found despite weeks of searching, officials can't be confident that they're looking for the black boxes in the right place, said Geoff Dell, discipline leader of accident investigation at Central Queensland University in Australia.
"They might be lucky and they might start smack bang right over the top of it," Dell said. "But my guess is that on the balance of probabilities, that's not going to be the case and they're in for a lengthy search."
The area where crews are looking for the devices lies within a larger 217,000-square-kilometer (84,000-square-mile) search zone that 14 planes and nine ships were crisscrossing Friday in the hopes of spotting debris on the ocean surface. The search zone is about 1,700 kilometers (1,100 miles) - or a 2 1/2-hour flight - northwest of the Australian west coast city of Perth. Several ships also had helicopters on board.
The search area has shifted each day, as the investigative team continues to analyze what little radar and satellite data is available while factoring in where any debris may have drifted due to ocean currents and weather.
"I think we've probably got to the end of the process of analysis," Houston said. "And my expectation is that we're into a situation where the data we've got is the data we've got and we'll proceed on the basis of that."
Houston said it was unlikely that any additional pinger locators would join the search any time soon because they are in scarce supply.
Although Australia is coordinating the ocean search, the investigation into the plane's disappearance ultimately remains Malaysia's responsibility, though Australia, the U.S., Britain and China have all agreed to be "accredited representatives" of the investigation, Houston said.
Four Australian investigators were in Kuala Lumpur to help with the investigation and ensure that information on the aircraft's likely flight path is fed back
to search crews, he said. The two countries are still working out who will be in charge of the analysis of any wreckage and flight recorders that may be found.
Australian Prime Minister Tony Abbott met with staff Friday at the Australian Maritime Safety Authority, which is running the search efforts, and acknowledged that officials have no idea how long the hunt would continue.
"It is probably the most difficult search that's ever been mounted," Abbott told staffers. "A large aircraft seems like something that would be easy enough to locate - but a large aircraft that all but disappeared, and disappeared into inaccessible oceans, is an extraordinary, extraordinary challenge that you're faced with."
Associated Press writers Eileen Ng and Gillian Wong in Kuala Lumpur, Malaysia, Rod McGuirk in Canberra, Australia, and Kristen Gelineau and Rohan Sullivan in Sydney contributed to this report.