I think it's a group; I think LEO knows this, and I think that everyone in the group has an airtight alibi for at least one of the shootings. I have been thinking at least three, but now I'm not sure. there could be a couple of shooters and several more doing planning, surveillance, and decoying, not to mention a network of homes and places to hide the van.
I've been thinking it was a cell for the last few days. I can't quite figure out the logic of using the same gun though. Any idea?