CHICAGO (AP) — It happened after Columbine, Virginia Tech, Aurora, Colo., and now Sandy Hook: 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.