all as part of every checklist. >> reporter: sources say malaysian authorities told the u.s. a cell tower in malaysia 250 miles from where the plane turned around picked up a signal suggests the co-pilot's phone was the only phone turned on after transponder turned off. u.s. officials caution there's no evidence the first officer nor anyone else on the plane tried to make a call. the twists and turns started early in the investigation. first, fears of terrorism with two passengers using stolen passports. >> the more information we get the more we're inclined to conclude it was not a terrorist incident. >> reporter: then after days of searching the south china sea say long the route the plane should have been the bomb shell. the plane had turned around, heading back over the malaysian peninsula. and after weeks of insisting the last words from the cockpit were all right, good night, malaysian authorities suddenly changing the wording to, good night, malaysian 370. and now they say those words