A senior U.S. defense official thinks the search for Malaysia Airlines Flight 370 could take years.  

The undersea drone Bluefin-21 is expected to complete the last of its 16-hour trips today scouring a small area of the southern Indian Ocean floor for the plane's wreckage.  

The drone has turned up nothing since it began searching two weeks ago.  

That means the search will have to move from an area authorities thought was their best bet for finding the missing jetliner out to broader ocean areas where the plane may have gone down.  

Flight 370 vanished on March 8 with 239 people on board.