Ragheads always bomb in the name of Satan/allah, no hint of that so probably not them. BLM is too stupid. That leaves Antifa, DNC, or FBI. Austin is the last place you'll find a lot of patriots, so hard to picture it as a leftard operation, unless they are after the Texas government. I'm going to go with targeted attack on at least one of the first people to be bombed, with subsequent bombs meant to muddy the water (or maybe the bomber just enjoys it too much to stop).
That's when EVERYTHING became "tamper-proof".
Chicago Tylenol Murders (Wiki)- Turns out the killer was never caught.
A lot of time and trouble but it could be a variation on this approach. Just saying.