The peak was apparently in 1929 and was trending down in the last few years, so increase is a hard thing to pin down.
I wish they had better historic stats on mental health, but something like 38 of the last 61 mass shooters showed signs of mental illness prior to their crimes. I think as Virginia Tech points out, any gun in the hands of sick person is worse than any gun in the hands of responsible people.
If you are serious about this do you have any ideas on how we address the mental health aspects?