It is funny watching the usual suspects dance around this one.
They can't play up the racial component because the racism is going
the wrong way.
They can't bring up the "gun show loophole" because this guy went
through all the hoops, although is does keep being brought up as
something he might have done ... but he didn't..
The fact is, he did go through all of the recommended "Brady"
processes and purchased these guns legally. It could have easily
happened in any state in the union. I have seen no reason why
Massachusetts, New York or New Jersey would have denied him.

Then we are left with the mental health issues. I have not heard about
him seeing a doctor and even if he did, would they have been willing
to release his medical records to the police?

That is the open question. How much does perceived public safety
allow the government to pierce doctor patient confidentiality?