the u.s. mexico border to fashion the biden administration's handling of immigration. as my colleague, morgan chesky reports, it happened just as the authorities announced a keyboard across and in arizona would reopen. >> reporter: gone patrol on the rio grande. the texas tactical marine unit giving nbc news an exclusive look on a stretch of river where debris litters banks, laden with barbed wire, where record migrant crossings have overwhelmed. there was no way to keep up? >> no. >> reporter: how quickly did this go from a mission of intercepting criminals to then saving lives? >> it was overnight. >> reporter: the migrant search is forcing staff sergeant lee ashcroft and his colleague to rescue more than arrest. >> reporter: how many extra life jackets are on board right now? >> 6. >> reporter: and if you come across a group of ten or more? how would you help everyone? >> it is tough. it's a tough decision. >> you got it, you've got it.