our breaking news tonight. the search is back on for flight 370. and wit everyone hopes only answers. complicating the effort determining how far that plane flew after running out of few. tom forman shows us how investigators have been doing that. >> reporter: in terms of a primary search area, we're no longer talking about millions of square miles, but about 621,000 square miles. how do you break down something that big, even with a lot of assets? this is how it's done. imagine our plane coming in here over this search area in the indian ocean, and this is what they essentially do, they will impose a grid on the ocean. but all of the squares of this searchable area are not equal in value. for example, in this case, you might say the most likely place is the middle, because that's simply where all the averages come together. that's where you have chosen this square.