The best approach for immediate support is a technique called psychological first aid, according to Robin Kerner, director of quality initiative and outcomes at St. Luke's-Roosevelt Hospital Center in New York City.
Rather than asking people to retell their traumatic stories, responders tend to the victims' immediate needs, reassure and comfort them and "perhaps, most importantly, connect them with their social supports.
"Research has shown that the retelling of the traumatic story in the immediate aftermath can lead to retraumatization and does not provide comfort to victims," said Kerner.
The probability of developing PTSD is increased if the victims had direct exposure or were seriously hurt or believed they or their families were in danger. Reactions such as crying, shaking, vomiting, feeling apart from their surroundings or helpless to get out, can be signals, Kerner said.
An earlier life-threatening event, a history of child abuse or mental problems raises the vulnerability level.
Those exposed to the movie theater shooting through images and reports on Twitter may see these posts and feel anxious or worried.
Viewing such events can be "disturbing if not dangerous" for young children, said Dr. Eugene Beresin, director of training in child and adolescent psychiatry at Massachusetts General Hospital in Boston.
Experts have studied the effects in similar situations: the Oklahoma City Bombing, the Challenger disaster and the 9/11 attack.
"There is considerable evidence that PTSD in kids may develop by watching such events in the media," said Beresin.
Community and family support is critical, and families must seek counseling, he said.
"Survivors, particularly parents, need to take care of themselves first ... If the family members want to help the survivors and others who are more stressed and wounded emotionally, they need to seek the best care for their own personal trauma."
No one is exempt from that emotional distress, say experts.
"One reason is that events such as this are a threat to our assumptive world," said Dr. David J. Schonfeld, director of the National Center for School Crisis and Bereavement at Cincinnati Children's Hospital Medical Center.
"Every day, we make assumptions about our safety and those we care about. Otherwise, we may become overwhelmed by the harsh reality that, at any point, tragedy can happen to those we love.
"When something like this event occurs, it forces us to acknowledge that these are assumptions and therefore may not be true. It leaves us feeling vulnerable and unsettled."