'For people in the US it's the unthinkable," says Dr Larry Aber, professor of psychology and public health at Columbia University's Mailman School of Public Health in New York. "In the past, we've had to stretch our imaginations to understand Belfast and Beirut."
Whatever the final death toll, he says, "there will be multiples of three or four times that of children and relatives who have lost loved ones, dispersed over three states and hundreds of school systems. The co-ordination required to conceptualise, develop and deliver the outreach for mental-health services for children and families is unprecedented on US soil."
Children's reactions will depend on the nature and intensity of their exposure to the atrocity. "You have the full range of exposures from very light to having parents killed or severely traumatised. Some children are very vulnerable," Aber says. "They may already have experienced separation. Their family supports may be weak or strong.
"We are counselling parents not to allow children to stay glued to television watching repeated images of the buildings blowing up and the planes going in. You can protect them from that."
As for toddlers, "younger children react in non-verbal ways, but they do react."
Aber says his own 10-year-old had trouble sleeping last week. "She was very worked up, but she wouldn't talk about it. It was necessary to spend more time with her. My son, on the other hand was worried about my flying anywhere on an airplane."
Some children will not react immediately, but weeks later may behave out of character - "maybe when they learn that a classmate's father has died, or when their mother is out of town and is delayed six hours by a rainstorm."
Among teenagers, "prior experience suggests that we may see an increase in reckless behaviour - drinking and driving, playing around with weapons or getting into fights. A small number may become militaristic and call for bombings on the alleged perpretators. A major problem for teenagers is that although they may feel fear, they will be reluctant to admit vulnerability. "You have to open up communication to allow teenagers to admit that they are scared without humiliating them."
