Officials leading the multinational search for Flight 370 said there was no specific information that led to the underwater devices being used for the first time, but that they were brought into the effort because there was nothing to lose.
An arduous weeks-long hunt has not turned up a single piece of wreckage that could have led the searchers to the plane and eventually to its black boxes, which contain key information about the flight.
Beacons in the black boxes emit "pings" so they can be more easily found. The beacons' batteries last about a month.
"No hard evidence has been found to date, so we have made the decision to search a sub-surface area on which the analysis has predicted MH370 is likely to have flown," Cmdr. Peter Leahy, the commander of military forces involved in the search, said in a statement.
Two ships with sophisticated equipment that can hear the pings made their way Friday along a 240-kilometer (150-mile) route investigators hope 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.
The head of the joint agency coordinating the search acknowledged that the search area was essentially just a best guess - and noted time is running out 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 towed a pinger locator from the U.S. Navy and the British navy's HMS Echo, equipped with similar gear, looked for the black boxes in an area 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.
Because the U.S. Navy's pinger locator can pick up black box 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 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 that the pinger locator must be dragged slowly through the water at just 1 to 5 knots, or 1 to 6 miles per hour.
The type of locator being used is a 70-centimeter (30-inch) -long, cylindrical microphone that is towed underwater in a grid pattern behind a ship. It's attached to about 6,100 meters (20,000 feet) of cable and is guided through the ocean depths by a yellow, triangular carrier with a shark fin on top. It looks like a stingray and has a wingspan of 1 meter (3 feet).
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 so far, officials can't be confident 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'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 crisscrossed Friday in the hopes of spotting debris on the ocean surface. The search zone is about 1,700 kilometers (1,100 miles) northwest of the Australian west coast city of Perth.
Fourteen aircraft and 11 ships were involved in Friday's search activities in the greater search areas, the coordination agency said. Ships sighted a number of objects in the area but none were associated with the missing plane, the agency said.
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.
Australia is coordinating the ocean search, and the investigation into the plane's disappearance is ultimately Malaysia's responsibility. Australia, the U.S., Britain and China have all agreed to be "accredited representatives" of the investigation.
Four Australian investigators were in Kuala Lumpur to help with the investigation and ensure information on the aircraft's likely flight path is fed back to search crews, Houston said. The two countries are still working out who will be in charge of analyzing any wreckage and flight recorders that may be found.
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.