Create a national registry of mentally ill people to prevent gun ownership/exposure

I'm neither a gun opponent, nor advocate, and I do support stricter gun laws on semi-auto weapons. However, it's become clear that mental illness is primarily behind the recent rash of mass-shootings. 

My solution is to create a national registry/database designated to mentally ill citizens for the purpose of preventing them gun ownership and exposure. This database could be structured similar to the sex offender database and bolstered by laws that restrict gun ownership in the homes of the mentally ill. If you're diagnosed, you're on the list. If you're not diagnosed and you display odd, threatening behavior, you should be subjected to a mandatory evaluation - if diagnosed, you're on the list.

We as a society would grow more aware of the mentally ill in our communities and have a place to go to research the history of such citizens. It would also make it much easier to restrict the mentallly ill from purchasing weapons.