What happened Friday morning in Newtow, Connecticut even has some experts disturbed.
Dr. Peter Langman is a licensed psychologist and author of the book "Why Kids Kill: Inside the Minds of School Shooters".
"Typically it's people killing their peers whether it's in high school or college so the fact that it was a man gunning down little kids makes it really upsetting," said Langman.
The last time a mass shooting in the United States involved such young victims was in 2006 when a gunman killed five girls in an Amish school house in Pennsylvania.
Langman says the impact of such a shooting is long lasting, especially for the students.
"They may be afraid to go back to school, they may have a lot of nightmares, they may have significant trauma that could last for a while," he said.
As to how parents can explain how something like this could happen Langman said, "We don't know so it's hard to tell parents what to say to their kids, they could say this person was a very troubled person, it depends on the age of the children the older they are the more you might be able to explain what it means to be troubled.”
Langman adds its important for parents to give their children comfort despite their own concerns.
"They can't promise they'll always be safe but parents can reassure kids that this was something that happened far away and person who did it is no longer alive so he is no longer a threat and schools generally are the safest place kids can be," said Langman.
For more information on warning signs, lessons learned from school shootings and a typology on school shooters, click here.