"Turning off the lights may trigger the re-experience of the trauma and cause the images and the intrusive thoughts to appear to people," Ragan said. "They will likely be physically restless, jumpy."
Survivor's guilt is another psychological problem many of the women who escaped the shooting alive may have to cope with, Emory's Kaslow said.
"Even though they will obviously feel relieved and grateful that they survived this random act by someone, there will be some sense of 'why wasn't it me?'" she said.
As for recovery, experts say, the shooting bystanders and witnesses must return to a normal life as quickly as possible.
"It turns out if you avoid the place where the incident occurred and your only memory of that place is traumatic, that seems to promote a sort of fixation on it," Ragan said.
"You want to make new memories when you're ready, you don't want to live your life with the gym being associated with the traumatic event."