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With nothing found while hunting a targeted search zone of more than 6 square miles, the undersea search for the missing Malaysian Airlines jet extended its range.

The high-tech underwater drone, Bluefin 21, on Monday was mapping areas of the Indian Ocean immediately next to a 6.2-mile search area identified after acoustic pings believed to be from the plane's black box flight data recorders were detected.

It was the sixteenth trip for the submarine, which has gone to depths of more than 2.8 miles.

Still, “no contacts of interest have been found to date, the Australian Maritime Safety Authority said in a statement Monday.

In addition to the underwater search, 9 military aircraft and 12 ships were doing a visual survey of an even wider area of the Indian Ocean.

Malaysian Airlines Flight MH370 vanished without a trace on March 8 as it flew from Kuala Lumpur to Beijing with 239 people on board. The search for MH370 is the longest and most expensive in aviation history, with ships and aircraft from some two dozen nations taking part.

The Bluefin-21 Autonomous Underwater Vehicle is craned over the side of the Australian Defence Vessel Ocean Shield in the southern Indian Ocean during the continuing search for the missing Malaysian Airlines flight MH370 in this picture released by the Australian Defence Force April 17, 2014.Australian Defence via Reuters

—Jeff Black