We always hear about the mass shootings in our country, in heartbreaking detail.
What we seldom hear about are the shootings that do not happen, because police and behavioral experts knew enough in advance to stop potential tragedies.
It does happen, and there have been many additions to the science over the years. Journalist Mark Follman tells the story in his book .
We get a peek into a seldom-glimpsed realm when the author pays a visit.