On Friday, December 14, 2012, a gunman killed 20 children and six adults at Sandy Hook Elementary School in Newtown, Connecticut. The assailant eventually apparently killed himself. You can read more about this story at CNN.com. This is the second shooting in the news in a brief period of time, the other occurring at a shopping mall in Portland, Oregon where 2 people were killed.
The horrifying and unpredictable nature of these events have the high potential to lead to a diagnosis of PTSD. You can learn more about what factors increase risk for PTSD following a traumatic event in this article from About.com. You can also learn more about the specific effect of a shooting on the development of PTSD symptoms here.