Breaking news into CNN. New evidence that Flight 370 had turned around and was traveling at a low altitude. This coming from the co-pilot's cell phone. It turns out it was on after the transponder stopped working. A U.S. official with first-hand knowledge of the investigation says Malaysia knows this because the phone was searching for service, bouncing off of a cell tower, after the jet lost contact. Coming up, we'll go live to the flight simulator for an explainer on that.

But first, today, a new and critical phase of the search for Flight 370, 39 days after it vanished and a week after the last apparent encounter with pings from the plane's black boxes. The man heading up the search accepted that the batteries are likely deadnow and called in the U.S. Navy's underwater drone, the Bluefin.

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ANGUS HOUSTON, JOINT AGENCY COORDINATION CHIEF: The deployment of the autonomous underwater vehicle has the potential to take us a further step towards visual identification since it offers a possible opportunity to detect debris from the aircraft on the ocean floor.

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KEILAR: The Bluefin uses sonar to map a section of the ocean never before explored. But this is just painfully slow here. It takes two hours to even get down there. It takes 16 hours to scour just 15 square miles before heading back to the surface. And the absence of any plane debris on the surface, well that means the visual searches are soon to be called off. This is despite the fact