All of the above, with the exception of aliens...unless your personal tinfoil hat goes that way, I guess. Doesn't take much more than slowing down and imagining a potential target and the most likely opposition would be. If the bunker is in the US, I'd wager on mercs first, then DS agents. Private armies for hire are a thing, everywhere. Either way, a fight in a confined space is brutal. Nowhere to run. We learned that clearing buildings. This is like that, but multiplied.