A reminder, since 2009, 21 mass shootings in the united states and all of those were the work of a lone gunman with lethal fire power and at least half of those gunman had a history of mental illness.... See More
A reminder, since 2009, 21 mass shootings in the united states and all of those were the work of a lone gunman with lethal fire power and at least half of those gunman had a history of mental illness. Of course in each of these tragedies people had to make lightning fast decisions that made a difference in their survival. Jim avila with new stories tonight with the way people at the navy yard stayed alive. Reporter: A new harsh reality for the few who went back to work at the navy yard today. Armed guards at the front gate and a new fear in the heart. There is an evil in this world and there is an evil in the population. But we just have to be aware. Received gunshot wound to the head. Reporter: Those who survived called on to make literal life and death decision during a 30 to 60 minute moment of crises. When I saw all these guns drawing and policemen saying run, run. So we keep running until we got into the other building. Reporter: That decision to act rather than shelter in place may have saved hassan's life because the lone gunman was on a mission to kill, entering the front door, carrying a duffle bag, shooting away. From there down to the third floor shooting into the food court cafeteria and finally to the stairwell in the first floor killing a security guard before taking his hand gun. About two seconds later i heard another really loud bang and I realized somebody is in here and they're shooting people. Reporter: John weaver, 13 year veteran of the navy yard was at his desk on the fourth floor when he saw all around him die. He looked like he was all business except he had his jaw clinched so you can see the ball of his jaw. When he looked at her he looked straight at her, clinched that jaw and boom. Reporter: In all 12 innocents murdered where they worked, none actually in the navy, all identified as civilians from age 46 to 73, all victims of evil. Those who survived cannot forget. Jim avila, abc news, washington.
This transcript has been automatically generated and may not be 100% accurate.