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A gunman opened fire at a midnight showing of The Dark Knight Rises early Friday morning in Aurora, Colo., killing 12 people and injuring 59 others. The suspect, James Holmes, 24, is in custody after eyewitnesses told police Holmes set off a smoke bomb inside the theater and then started shooting.

Movie attendees described the scene as “chaotic” and “terrifying,” as they dropped to the floor and hid behind seats or ran for their lives, while other audience members were shot around them.

In the coming weeks and months, as the community focuses on moving past the event and healing, witnesses may face a number of mental, emotional and psychological hurdles, in addition to any physical injuries they may have suffered, said one psychiatry expert.

“For the victims, [the fallout] is going to be tremendous,” Dr. Michael First, a professor of clinical psychiatry at Columbia University Medical Center, told Fox News.  “There are a lot of people who were exposed to an unspeakably horrible event…and with the randomness of it, I suspect the vast majority were literally fearing for their own lives.”

Among other conditions, First said people could develop depression or substance abuse disorders, as well as post-traumatic stress disorder (PTSD).

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“The most common reaction to a traumatic experience is not PTSD [as people may think], but actually the increased use of alcohol or other drugs, and depression,” First said.  “It’s understandable; people want to numb themselves and self-medicate.”

In addition, for anyone who was already suffering from a mental or physical condition, such as depression, the event could exacerbate it.

“Most mental disorders can be made worse with stress, and this is about as stressful as an experience as anyone could have,” First said.  “There’s a strong link between the mind and the body, so I think people can expect a rough time after this and need to be on the lookout for any condition being worse.”

A smaller proportion of the witnesses may go onto develop PTSD, a debilitating anxiety disorder triggered by exposure to a traumatic experience.  A person’s likelihood of developing PTSD is mainly due to their own predisposition to the disorder, according to First.  The risk may be greater if the person has a family or personal history of mood and anxiety disorders.

“A lot of it is just natural factors -- certain people are just more predisposed,” First said.  “We can’t predict in advance who’s going to develop it, but we do know only a minority of people do, so that’s the good news.”

It’s important not to confuse normal reactions to a traumatic event with something more serious, like PTSD, First added.

“It’s very normal for people to have a bad reaction after trauma like this,” he said.  “People shouldn’t get too upset if they are having bad dreams and can’t get [the event] out of their head or have trouble functioning in the days after.  That’s not a warning sign for PTSD;  that’s normal.  In a severe trauma, it’s normal to have a severe reaction.”

However, if these trauma symptoms persist for months afterward, it is time to seek help.

“Part of it is re-experiencing the event, having nightmares and having the thoughts come back again and again,” First said.

Some witnesses may end up so frightened and determined to avoid the event, they may be unable to leave their houses.  “They may avoid anything that reminds them of the event, so any loud sounds, the TV or news,” First said.  “Certainly for people living in that area, the only way to deal with it is to stay in their rooms.”

In addition, it may actually make matters worse if psychologists, reporters or relatives try to ‘de-brief’ witnesses and get them to talk extensively about what happened in the 24 hours immediately following the event, according to First.

“People should not be forced to talk about what happened to them because what that actually does is it makes it more likely to fix that memory in their head,” First said.  “PTSD is actually sort of a traumatic memory that gets stuck and people can’t get over it.  If you immediately start talking about it, it’s actually more likely to get stuck.”

The most effective treatment for PTSD is behavioral therapy, which gradually desensitizes a person to the experience of the event.

“You try to get them in a very graded way to start thinking about it and eventually desensitize them to the anxiety,” First said.  “The key thing is getting them to function better.  If someone is in the house and can’t get out, and their life is on hold, with therapy you try to get them to slowly reintegrate themselves back into society and deal with the things they’ve been avoiding.”

For friends and relatives of the witnesses, First said the best way to help them cope and heal is “being there for them and trying to empathize.  Don’t force someone to tell the story, but if they want a shoulder to cry on, to empathize with how horrible it was, just being available for a lot of people is sufficient.”