CHICAGO – 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.

But warning signs “only become crystal clear in the aftermath,” said James Alan Fox, a Northeastern University criminology professor who has studied and written about mass killings.