heather: armed with an arsenal to kill, investigators trying to piece together the heart stopping moment of horror as they unfolded in las vegas. rob: how stephen paddock was able to stockpile dozens of deadly weapons with anyone noticing. more on the push to ramp up hotel security in the wake of this killing. >> reporter: we went to mandalay bay to see what it was like, went to an area of shops that connects to hotels, we walked through that area into mandalay bay, you saw people gambling and eating and seemed like business as usual. we didn't run into much security until we got to the elevator