deliberately brought down flight 370. >> they just don't fallout of the sky and it it would really take multiple failures of multiple systems for know believe that this is what could have occurred. >> reporter: the idea of a deliberate act is not being ruled out by the u.s. in part because of what evidence has not emerged. if the plane was in trouble, experts say, there should have been time for the pilots to send a mayday after the plane turned and flew west, but no call was heard. so was it a terrorist attack? u.s. officils say there's been no claim of responsibility so for now they don't think it's a likely scenario. what about a hijacking? again, experts say there likely would have been time for the pilots to make a call. so why not some kind of mechanical failure? >> if you had an emergency, if you had an explosive decompression, if you had a fire, those are catastrophic events that you want to get that