I do recall hearing that Klebolds folks were active in HCI. I recall talking about that with the father of one of the victims from the library and how ironic that seemed. I have no source to verify that however.
The don't trust others with guns because the don't trust themselves. And, perhaps, they have good reason.
There are connections between the shootings in the links below, to the school shootings and as far as I know these have not been solved.