Youth violence has become a vast subject of research, especially since the shootings at Columbine High School in Colorado in 1999, a seminal American crime event. School shootings make headlines, but some advocates have long argued that American schools–populated by 55 million students in grades K-12 and another 15 million in colleges and universities–are very safe, despite the deserved attention given to mass murders at Columbine and other schools, including the April 2007 shooting by an alienated student on the Virginia Tech campus that left 33 people dead. This source guide includes a wide array of contacts, from government and advocacy groups to experts on gangs and the post-traumatic stress disorder that typically follows school shootings. The website of the federal Office of Juvenile Justice and Delinquency Prevention, linked below, has a trove of reports, studies and statistics on the subjects. The federal Centers for Disease Control is another good place to start for basic background facts. The CDC's Youth Violence Prevention webpage, linked below, points out, for example, that a quarter of American students report gang activities at their schools, and that 38 percent of all public schools reported at least one violent incident in 2006.