Witnesses at Sandy Hook Elementary School reported horrific scenes as a shooter took 27 lives Friday -- the shattering sounds of gunshots, children locked in the bathrooms and parents crying outside in the parking lot.
Experts say that the young children who saw events first-hand can have lasting psychological scars, but those whose home lives are stable and supportive will have fewer long-term scars.
"It was horrific," said Kaitlin Roig, a 29-year-old teacher, who was in a morning meeting when the gunman entered the school.
"Suddenly, I heard rapid fire, like an assault weapon," the first grade teacher told ABC. She rounded up her 14 students and locked them and herself in the bathroom. "I helped kids climb on the toilet dispenser [so they could all fit in].
"I thought we were going to die."
Children in such a situation "are terrified, and they don't have the cognitive or emotional capacities to make sense of this," said Dr. Nadine Kaslow, professor and vice chair of the department of psychiatry at Emory School of Medicine.
"Not that any of us can make any sense of this," said Kaslow. "It's truly inconceivable."
At least 27 people, mostly children under the age of 10, were shot and killed at the K-to-4 school Friday morning, federal and state sources tell ABC News.
The massacre drew SWAT teams to the school and the town of Newtown locked down all its schools, authorities said today.
According to federal sources, the gunman was identified as Adam Lanza, 20. His mother was one of the victims.
One mother named Christine who has a child at Sandy Hook told ABC about the chaos that ensued when she arrived at the school Friday.
"When I got there, there were just parents running into the firehouse because they were directing us there. That's where children had been evacuating to, and we went in and people were just grabbing their children and hugging and crying. There were lots of children crying."
She said another parent who had been at the school at the time was "pretty broken up." Many parents didn't know where their children were.
In 1996 in Dunblane, Scotland, 15 children and a teacher were killed in a similar massacre.
Parents and caregivers play the most important role in a child's recovery from a traumatic event, according to Dr. Gene Beresin, director of training in child and adolescent psychiatry at Massachusetts General Hospital.
"Children need to know that they are safe," he said. "Are people taking care of me? How is this going to affect my life? They need to be reassured."
"Thinking about kids in all disasters, you think about the airlines -- when the oxygen mask drops, you put your mask on first and then help the child next," said Beresin.
"Parents need to take care of themselves first. [The children] need to know you are calm and in control," he said.
Adults and community support is critical, according to Beresin.
Young children who witness violence can have acute or post-traumatic stress disorder. "The immediate reaction is shock and horror," he said.
After events like this, communities typically set up crisis centers in a church or other public place where people can seek professional and spiritual help.
Turn the television off, say experts, but answer your children's questions. Don't disregard an older sibling who is watching the news unfold and is worried. They need assurance, too, he said.
According to Beresin, young children may not have "discreet memories" of the event, but they can still have an emotional reaction, experiencing nightmares or, conversely, emotional numbing, said Beresin.
"Some kids shut down," he said. "They may actually turn off and not want to be hugged or cuddled -- that's a normal response. Some kids are clingy, and others will withdraw."
Kids can also regress in the aftermath of a traumatic event.
Parents should not force a child to open up, but "don't let them be alone," he said.
One way young children can work out problems are through reenactment. "They may be playing a game about shooting and dying, and parents should not stop that," said Beresin. "Let them do it."
Young children can also ask questions that don't directly relate to the event, according to Rahill Briggs, assistant professor of pediatrics at Montefiore Medical Center in New York City.
"They can ask directly or less directly about guns, or heaven or death or about a pet that died," she said.
In studies of 9/11 one of the findings -- not a surprising one -- after the terrorist attacks was that those who were most directly affected "suffered the most," according to Briggs. Coping with grief long-term depended on the cohesion of the child's family -- "how well the caregiving system responds to distress. When it is proactive, by definition the children do better."
"What was the most incredibly predictive five years out was how everyone was doing before the incident," said Briggs. "It is the same for mental health in general, those who are coping well in their lives before a trauma are the most likely to cope well afterwards -- even if they saw the towers fall."