Mercer was booted from the military and had a suicide attempt. Data mining can stop some crimes--but police still have to do the leg work.
"A software program alerts police to a social-media posting by an individual of interest in their jurisdiction. An algorithm reminds them why the individual had become a person of interest—a history of mental illness, an episode involving a neighbor. Months earlier, discreet inquires by police had revealed an unhealthy obsession with weapons—key word, unhealthy. There’s no reason why gun owners, range operators and firearms dealers shouldn’t be a source of information for local police seeking information about who might merit special attention."