This is interesting stuff. 

I have been collecting American mass murders for a year and a half now, starting in 1657.  I have identified 504 incidents with 6568 dead, d I know of least 300 more mass murders to add. (Definition: Three or more murdered in a 24 hour period, and of course excluding acts of war by state actors.)  Just over 26% of these mass murders have either severe mental illness or likely mental ill as the proximate cause.  The attached graphs come from my spreadsheet of horror.  You will notice two major bursts of mental illness caused mass murders: one in the 1860s and 1870s coincident with our first war that produced noticeable PTSD.  The second rise is after the state mental hospital system was destroyed in the 1960s and 1970s.  Of course, this includes all mass murders, of which less than half involve firearms.  Axes, hatchets, knives, and poison have always been very popular.

I do believe this study is going to be very enlightening. Keep an eye on Clayton.