Chilling New Video of the Navy Yard Shooter

Aaron Alexis marched through the Navy Yard going door to door with a sawed-off shotgun in his hand.
2:27 | 09/25/13

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Transcript for Chilling New Video of the Navy Yard Shooter
We begin with a haunting new video, a killer roaming the halls of the washington navy yard looking for targets. The fbi released the new surveillance tape showing frame by frame how the gunman moved in secret and around him workers scramble for cover. The video is not easy to watch but abc junior justice correspondent shows us what it reveals behind the mind of a mass shooter. Reporter: This is alexis armed with a sawed off shotgun hunting innocent victims. It started as a routine monday at the navy yard. Alexis shows his pass and is waved in by a security guard, just before 8 am. Surveillance cameras capture him driving into a parking garage and moments later walking in through the front door of building 197. He's been hired as a civilian contractor working on the building's computer network. There is no metal detector or bag check. We next see him armed with a shotgun, peeking behind doors, look be for someone, anyone to ambush. He moves quickly down the stairs with purpose, eager. Look, he sees some people and ducks behind a wall. Today the first official explanation of why. It was a mentally unable individual. Reporter: This shotgun is the primary murder weapon. On it he etched a number of carvings including end to the torment. Alexis had been hearing voices and believed he was being bombarded by micro waves, something the navy knew about. He held a delusional belief that he was being controlled or influenced by extremely low frequency or e.L.F. Electro magnetic waves. Reporter: In his computer files the fbi found this. A document retrieved from the electronic media stated quote, ultra low frequency attack is what I've been subject to for the last three months. And to be perfectly honest, that is what has driven me to this. Reporter: So what should have been a routine day, alexis killed 12 people, mothers, fathers, sons, daughters. Diane, they never had a chance and still no answers to why so many red flags were missed. Thank you, pierre. And also next tonight we have new details about that

This transcript has been automatically generated and may not be 100% accurate.

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