Every time I thought I knew what was going on, somebody would say something that would make me rethink it. 'Well, obviously it's-- Yeah, except it can't be because...' Near the end, though, there was only one possible answer. Only one person could have done it, even if I did suspect, until the last, that they'd had someone else's assistance.
Damn, dude, well done. And I went through two endings (I'm pretty sure more were possible, but i don't have a burning completionist urge to be intentionally wrong), because I was sure there was a better one, if I just had that one conversation a little differently.