Here is the best single source I could find. It is from Mother Jones, a liberal source so take it for what it is. But it provides a fairly comprehensive historical reference (time line).
Questions it doesn't answer:
How many of these mass shooters are from broken, dysfunctional or single parent households ?
How many of these mass shooters, especially the more recent, were hardcore gamers?
Are shootings being accelerated because of a prolonged economic slump?
http://www.motherjones.com/politics/...-shootings-map
