PERTH, Australia —
A ship-towed device detected four signals underwater that are believed to have come from the airliner's black boxes shortly before the beacons' batteries died. The sounds helped narrow the search area to the waters where the Bluefin 21 is now operating.
The U.S. Navy's unmanned sub cut short its first mission on Monday because it exceeded its maximum operating depth of 4,500 meters (15,000 feet). Searchers moved it away from the deepest waters before redeploying the sub to scan the seabed with sonar to map a potential debris field.
On the ocean surface, up to 14 planes and 11 ships were searching a 62,000-square-kilometer (24,000-square-mile) patch of sea about 2,200 kilometers (1,400 miles) northwest of Perth on Wednesday. The surface search is expected to end soon as not a single piece of debris connected to the plane has been found.
Investigators are also waiting on test results from an oil slick found about 5,500 meters (3.4 miles) from where the underwater sounds were detected.
In addition to finding the plane itself, investigators want to recover the black boxes in hopes the cockpit voice and flight data recorders contain answers to why the plane lost communications and flew so far off-course before disappearing.