For balance:
A report I heard on the evening of the 21st of June said that according to then-current information all his firearms had been bought legally. It was later confirmed separately. However, as Decius notes below, it is unlikely that he used a fully automatic firearm, as the difficulty in acquiring one makes it more likely that if the man used a fully automatic weapon, it was an illegally modified semi-automatic.
Disinformation is NOT noise. I included 4 adjective phrases in my claim, and I know that the claim is false considering any combination of only three of them. The sources you linked choose explicitly to not state if the firearms in question are fully-automatic, but the shops listed typically don't make five-figure sales, like a fully-auto AR-15.
For reference, the least expensive firearm matching the 'fully-automatic' and 'legal in the US' qualifiers are around $2500 each if the seller needs to sell them within hours, and $4000 if the you want to buy one in a reasonable amount of time.
A Ph.D student in neuroscience shot at least 50 people at a showing of the new Batman movie. He also appears to have released some kind of gas from a canister. Because of his educational background this person almost certainly knows a lot about molecular biology. How long will it be (if ever) before a typical bio-science Ph.D will have the capacity to kill, say,a million people?
Edit: I'm not claiming that this event should cause a fully informed person to update on anything. Rather I was hoping that readers of this blog with strong life-science backgrounds could provide information that would help me and other interested readers assess the probability of future risks. Since this blog often deals with catastrophic risks and the social harms of irrationality and given that the events I described will likely dominate the U.S. news media for a few days I thought my question worth asking. Given the post's Karma rating (currently -4), however, I will update my beliefs about what constitutes an appropriate discussion post.