What's interesting is Michael Moore, who spent a lot of time trying to figure out Columbine (and made a movie about it), has been doing a lot to promote this theory. In an advocacy film I saw on the issue, he said he could never figure it out while making Bowling for Columbine what caused mass shootings and now he's convinced it's the pharmaceuticals. I tend to see a lot of wisdom in these arguments.
Almost every mass shooting was committed by a CRAZY sob that had no business possesing a gun, let alone a bottle of water. More gun laws will accomplish nothing, but cause the "rest of us" that abide by laws, fill out forms, and are subject to background checks be be treated like criminals.
This issue is NOT about preventing shooting crimes. It' about Gov't control. They want you a SUBJECT, a tax payer, not a citizen.