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(CNN) – Searchers are under the gun. Malaysia Airlines Flight 370's flight data and cockpit voice recorders' pinger batteries are already two days past their required shelf life.
There is no guarantee from this point on how much longer they will last if they have not died already, but here's why authorities are now "cautiously optimistic" – while there's still no wreckage, there was a distinct sound coming from underwater, and searchers are trying to determine if it is Flight 370's black boxes that are calling.
CNN's René Marsh reports.