Nightly News   |  September 25, 2013

Eerie video shows Navy Yard shooter hunting for victims

The FBI has released surveillance video that shows Aaron Alexis inside the building where the shooting happened, carrying a shotgun. NBC’s Pete Williams reports.

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>>> has released the video that was recorded inside the washington navy yard complex showing the gunman going about his attack. we have also learned more about the clues left behind that speak to the depth of his mental illness at the time. the victims' loved ones have seen the video you are about to see. it shows him stalking the hallways before the hail of gunfire that resulted in 13 victims, including the gunman. our justice correspondent pete williams starts us off tonight in our d.c. newsroom. pete, good evening.

>> reporter: good evening. investigators say aaron alexis believe he was torment ed by a form of radio waves once use bid the navnavy. cameras inside the navy yard building show alexis carrying the sawed-off shotgun he used to shoot most of his victims.

>> mr. alexis was a mentally unstable individual. he was able to obtain a firearm two days before he carried out an attack that was planned to kill as many innocent siecivilians as possible.

>> reporter: pictures released today show alexis arriving at 7:53 a.m . driving his rental car into a parking deck. at 8:08 he walks in into building 17 carrying a backpack presumably with the gun and ammunition inside. he emerges from the restroom, holding the shotgun, looking for victims, looking into office doors. another camera catches him coming down a stairway t with the shotgun. fired at victims on at least three floors of the building. the last video shows him in another hallway, hiding behind a corner as people rush away at the end of the corridor. he was killed at 9:25. he scratched into the barrel of the shotgun, end to the torment. on the body of the gun, not what y'all say, better off this way and my elf weapon.

>> alexis held a belief he was being controlled or influenced by extremely low frequency or e.l.f. lekt magnetic waves .

>> reporter: the navy used e.l.f. to communicate with submarines but shut the program down a decade ago. the fbi says a search of alexis 's backpack turned up something he wrote. ultra low frequency attack is what i have been subject to for the last three months. to be perfectly honest, that is what has driven many to this. investigators say there is no evidence that alexis called or e-mailed anyone to warn of the attack. they found no indication that any workplace dispute set him off. brian?

>> pete williams start thing us off from washington. thanks.