Shooting erupted at the midnight premiere showing of "Dark Knight Rises" in an Aurora, Colo., theater complex filled with young adults, teenagers and young children with their families, some dressed in playful Batman-genre costumes.
Everyone expected a night of fun, not a massacre. Smoke bombs went off. A gunman stalked victims in the aisles, killing at least 12 people, including a 3-year-old. Witnesses said blood was everywhere.
The surprise, as well as the magnitude of the mass shooting -- with 71 injured or dead, the largest in U.S. history -- was enough to trigger post-traumatic stress symptoms in those who were vulnerable, said Dr. Jeffrey A. Lieberman, psychiatrist in chief at New York Presbyterian Hospital-Columbia University Medical Center.
"On the emotional Richter scale it was very high," he said. "You go to a movie like Batman because it's fun-loving entertainment, and you are seeing kids in costumes and the last thought you are thinking about is some type of seriously dangerous, potentially life-threatening situation. The contrast adds to the potential for emotional trauma."
One witness told ABCNews.com, "You just smelled smoke and you just kept hearing it. You just heard bam bam bam, nonstop. "The gunman never had to reload. Shots just kept going, kept going, kept going."
For continuing coverage of the Tragedy in Colorado: The Batman Massacre, tune in to "World News," "20/20" and "Nightline."
Psychiatric experts said it was hard to know who would experience serious after-effects of the attack. Only about 7 to 8 percent of all individuals will go on to develop post-traumatic stress disorder, or PTSD, after such an event, according to the Dart Center for Journalism and Trauma.
Most people, if they are not exposed to repeated trauma like war, are resilient and have extraordinary coping skills. But those who are vulnerable can have lifelong effects, said experts.
"We all have our breaking points," said Lieberman. "Everyone, given sufficient stress, like prisoners of war, have different levels of endurance. But events have a residual effect."
Nine miles away in 1999, among those who witnessed the slaughter of 13 students and teachers and the suicide of two shooters at Columbine High School in Littleton, Colo., a handful went on to experience repeated nightmares, flashbacks and anxiety-related disorders.
"Even for those people who were not affected, these are peripheral events for people who live in the town and in the state, and they can have an identification from the geography and connection to this," said Lieberman.
About 10 percent of women develop PTSD sometime in their lives compared with 5 percent of men -- about 5.2 million adults in a given year, according to the Department of Veterans Affairs.
Children, it seems, are more resilient than adults.
"It has an impact on them," said Lieberman. "But they have in place readily defined support systems in family and school social structures."
Those who are most prone to PTSD were directly exposed to a traumatic event -- they were either victims or witnesses, or were seriously injured. But one study after the 9/11 attacks found rippling effects on witnesses.
At one school two blocks from the World Trade Center, about 27 percent of staff members who saw a plane fly into one of the towers lost time from work because of physical symptoms, according to a Centers for Disease Control and Prevention study. About one-third reported symptoms consistent with depression.
The degree to which people are affected is determined by their proximity and how sustained or horrific their exposure; their own psychological make-up and the help they receive after the event.
"Your individual vulnerability and resilience is determined by your genetic make-up, and also in part by the psychological features you have developed over the course of your lifetime -- were you confident and successful and could you overcome experiences, or were you cautious and fearful?" said Lieberman.