A primary thread to look for in the mass shootings is that the lion's share of the official focus is placed on a mind-controlled (feeble or frail minded, abused/harassed, drugged, etc.) patsy while a larger operation, the "real operation," plays out with the patsy's mythology as its cover. What this generally translates to is significant witness reports of other shooters and/or a framework in which the patsy's sometimes outlandish behavior ahead of time (the establishment of a "legend," e. g. Tim McVeigh's distribution of The Turner Diaries, Lee Harvey Oswald's supposed Marxism) is known to authorities but specially protected by those on the inside with an allegiance to the syndicate which is ultimately responsible for the attack. (This allegiance may be overt and consciously conspiratorial or more of an unspoken or cultural directive.)
In the case of the Columbine shootings, this would translate to Eric Harris' widespread Jekyll-and-Hyde reputation as a nice guy/psycho among his classmates and school disciplinary staff and the many independently corroborated witness reports of up to about 10 shooters, including several similar "known psychoes" in the trench coat group Harris and Klebold were on the fringes of. These other individuals were named by name in a significant number of instances. There also seems to be at least a third tier to the Columbine attack in which at least one adult was reported as a shooter.
"The Columbine Cause"
Other ones to check out include Port Arthur (Tasmania 1996):
A Question of Guilt: The Massacre at Port Arthur Video - sevenload
and Dunblane (UK 1996):
The Fort Hood shooting (Texas 2009) was, indeed, very suspicious as well. It more or less follows the pattern discussed above to a tee:
Fort Hood Shooting, Killeen, Texas, 5 November 2009 - OTHER STUFF - ODDS AND ENDS - Echoes of Columbine - Message Board Yuku