Purely my opinion and speculation here, but I'd say the things that stick out to me are the timing and avoidance of random casualties for one. This is a hallmark used by the IRA and the ETA in Spain to inflict damage upon the government without damaging the people, so to speak. The use of an RV also stands out to me, I don't recall many VBIEDs associated with foreign terrorists using that type of vehicle in the US, while I do recall RVs being a common tool in militia groups for movement, travel, storage, etc. Some other things stuck out to me watching videos of the explosion, but it's hard to say anything definitive until more evidence is released.
What would you consider the IRA to be in lieu of anti-government?
I don't think I said the IRA were a militia, I drew a conclusion that this incident, which I speculate as being tied to anti government militia groups in the US, shares similarities with other destructive anti government groups seen in Ireland and Spain. I don't think I said those groups were militia groups specifically, and I don't think the comparison is such a stretch since there are other commenters making the exact same leap to the IRA.
My contradictory wording is likely to be a subconscious defense mechanism of sharing my opinion on the internet. I have not claimed any specific knowledge or authority over this event or even in defense of my own speculation here. In full transparency, the only reason I posted was because people were going all in on the "heist" theory, which I find ridiculous.