Josh Klapow, Ph.D., associate professor in the UAB School of Public Health, Clinical Psychologist, Chief Behavioral Scientist for ChipRewards, Inc.

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• Klapow is available for phone interviews as well.

From CNN.com Smith told CNN that person was her 18-year-old son, Mathew. She said she didn’t want to reveal his last name.Smith said that after three people were shot and killed, the gunman paused and spotted her son. "Hey you, with the glasses," he called out."The shooter asked him to give the police something, and that if he did, he would live," Smith said."Mathew said at that point, he didn't quite get what the shooter said. He thought he was standing up to die, and that when the shooter gave him what he was told to give to police, he was then sent to sit in the back of the room ... and to watch what was going on."Her son did as he was told, she said."Mathew said that he froze. He didn't make a single move. He was afraid to look away. That if he ... did anything to make the shooter notice him, that he would be shot. So he just sat there," Smith said."He's sitting there, watching the shooter execute people?" asked CNN's Dan Simon."Yes."Mathew never saw the contents of the envelope but could feel a computer flash drive inside, his mother said. He gave it to police.

Josh’s ResponseMathew has just survived something that many people could not even imagine. He has been exposed to a life threatening situation that is far outside the realm of normal human existence. His reactions following these events may range from a broad array of acute stress reaction symptoms (nightmares, intrusive thoughts, severe anxiety, hyper vigilance) to complete emotional numbness as his natural and hard wired psychological defense mechanisms kick in. For Mathew, right now, psychologically he needs to be supported. It’s almost akin to an individual with a severe infection whose body needs time to develop the strength and the antibodies to fight it off. He may feel nothing, he may feel extremely scared, he may feel extreme guilt and also extreme relief.

What can he do? What can others do?

He must be under the care of a team of mental health professionals. Mostly focused on watching his symptoms and giving him ways to cope minute by minute, day by day. He needs to know that he has access to professionals who are equipped to help him through this. This is different from friends and family. He needs them as well. The most important thing people can do for Mathew is work with him and not try to move him emotionally in one direction or another. This is about being thee to support him, to listen if he wants to talk, to leave him alone if he needs time to process. His needs will change day by day, even hour by hour. Letting him know that he has unconditional support, that people are there as his friends and family AND that he is under the watchful eye and care of a mental health team will allow him to process this tragic event.

The key is that this is a process. His brain needs time to synthesize the information, to make some sense of it, and to incorporate this event into how he lives his life. This is not days, this is weeks, months, years.