FILE - In this Friday, Dec. 14, 2012 file photo, parents leave a staging area after being reunited with their children following a shooting at the Sandy Hook Elementary School in Newtown, Conn., where Adam Lanza opened fatally shot 27 people, including 20 children. People figure there surely were signs of impending violence. But experts say predicting who will be the next mass shooter is virtually impossible _ partly because as commonplace as these calamities seem, they are relatively rare crimes. Still, a combination of risk factors in troubled kids or adults including drug use and easy access to guns can increase the likelihood of violence, experts say. (AP Photo/Jessica Hill, File)

FILE - This undated photo shows Adam Lanza in a Newtown High School yearbook photo. Authorities have identified Lanza as the gunman opened fire Friday, Dec. 14, 2012, inside an elementary school in Newtown, Conn., killing 26 people, including 20 children, before killing himself. People figure there surely were signs of impending violence. But experts say predicting who will be the next mass shooter is virtually impossible _ partly because as commonplace as these calamities seem, they are relatively rare crimes. Still, a combination of risk factors in troubled kids or adults including drug use and easy access to guns can increase the likelihood of violence, experts say. (AP Photo)

FILE - In this July 23, 2012 file photo, James E. Holmes appears in Arapahoe County District Court in Centennial, Colo. Holmes was being held on suspicion of first-degree murder, and facing additional counts of aggravated assault and weapons violations stemming from a mass shooting in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. People figure there surely were signs of impending violence. But experts say predicting who will be the next mass shooter is virtually impossible _ partly because as commonplace as these calamities seem, they are relatively rare crimes. Still, a combination of risk factors in troubled kids or adults including drug use and easy access to guns can increase the likelihood of violence, experts say. (AP Photo/Denver Post, RJ Sangosti, Pool, File)