I can't quote any studies on the subject but it seems to me that most of the individuals engaged in these acts in teh past have given many years notice that they were as barmy as a box of balm. If your objective is to stop gun-armed loons then irrespective of the general laws on firearms would it not be sensible to have some cross-checking against a well-prepared database of crackpots? Woud it not also be a wise plan to go further and to ensure better provision of better quality _borderline_ nut-care?
Indeed, but what are you going to do practically ?. The only thing that would stop a potential nut case is incarceration. Psychology teaches us that certain people are a greater risk factor than others, but we can't jail people for what we think they might do, correct me if i'm wrong, gun laws in the US already prohibit you from owning a gun if you have a diagnosed serious mental disorder. If society is completely saturated with firearms it makes it that much easier for anyone to get one.
If I wanted a pistol I would know who to ask, but it's doubtful that he would be able to come through, and even then the cost would be astronomical.
He didn't bring the gun with him on the plane from China though.