The other day I posted an idea about a "red-flag" system, similar to a restraining order, that could have prevented mass shootings where the perpetrator was known to authorities in advance:
http://www.democraticunderground.com/10027935828
It turns out my idea is not original, and I really think that this is something we should be promoting:
"The GVRO process is similar to the domestic violence restraining order process and provides the opportunity to defuse a potentially dangerous situation before permanent harm is done. Under a GVRO policy, family members and/or law enforcement officials can petition a judge to have firearms temporarily removed from an individual in crisis for a period of one year. This would allow those who are a threat to themselves and/or others the time they need to get help and heal. Equally important, the GVRO allows for due process, as individuals are able to petition to get their firearms back following a brief period of time."
http://www.huffingtonpost.com/josh-horwitz/a-gun-law-that-could-have_b_10610360.html
The Aurora police had the Tarasoff letter from Holmes' psychiatrist; Pima college police knew Loughner to be a danger. Similar conditions existed for most of the major shootings.