James Holmes was meticulous in his preparations. He shopped at different gun stores and hid his online purchases from colleagues and neighbors. It may well be that no gun ban or background check could prevent such a determined man--one with no criminal record and with uncommon student achievement in neuroscience--from acquiring assault weapons. And so the Holmes case raises a crucial question: Is there a way to identify and stop mass killers before they unleash themselves?
The study of mass murderers is at once fascinating and frustrating. After Columbine, the Secret Service and the FBI undertook months-long projects that were designed...