>> reporter: the search for flight 370 has entered fourth day. dozens of ships and helicopters and planes running grid like searches in the waters between malaysia and vietnam. so far, nothing. an oil sheen and debris spotted over the weekend ended up being false leads an now sea conditions are expected to worsen. on "blue ridge" the commander. >> that will make search conditions more challenging but i can tell you our radars are very advanced and they can actually negate a lot of that surface clutter, but it does present a challenge for us big international effort. >> reporter: the last reported location for the boeing 777 was over the water halfway between malaysia and vietnam, but with malaysian military suggesting the plane may have started to turn back, the search zone is expanding mailings mainland and waters to the northwest. >> that is very important. >> reporter: among the questions why would the plane turn back? and why didn't the crew radio air traffic controllers?