seoul u.s. military say saturday open investigation fake mobile phone alert social media message advise u.s. military personnel families evacuate korean peninsula phony message spread thursday come sensitive time tensions high north korea conduct sixth large nuclear test sept. 3. test series missile launch trigger war word u.s. president donald trump north korea leader kim jong un u.s. force korea usfk yet aware many people receive hoax message behind spokesman usfk tell reuters usfk post statement official facebook page thursday make clear u.s. military issue evacuation-related alert also instruct u.s. department defense personnel family confirm evacuation-related communications act usfk conduct regular biannual noncombatant evacuation operation neo exercise spring fall rehearse possible evacuation second neo exercise year schedule take place october u.s. army probe fake evacuation order send u.s. military families south korea