Is it Katherine or maybe Tatiah ?

Is it Katherine or Tatiah was the one who killed Brian ?
I don't think it's Elena . And I think Katherine and Tatiah has the same fingerprint with Elena, right ? since Elena is their dopplenganger, I think so :/

I know people might think it's more likely Katherine, but think about it . In Bringing Out the Dead, Klaus and Elijah told Damon and Stefan about Tatiah. Maybe, just maybe, the writers tried to give us a hint that Tatiah probably still out there, and just waiting for the right time to make an entrance. And I read somewhere that Brian's death was pretty important for the storyline.

So ? what do you think ? I know this is sound confusing, but I just want to tell you about my theory :D