Who kills the 11th Doctor in the episode "The Impossible Astronaut"?

We know that the little girl was in the spacesuit in 1969 but she was no longer in it by the end of that 2 part episode. So how do we know it was the little girl in the spacesuit in 2011. It could be anyone in the space suit, couldn't it? Anyone who would know that if you kill a time lord while they're regenerating that they will die forever.