But that raises the troubling question of why Dumbledore let Sirius go to Azkaban, knowing he was innocent. The question can be answered in a few different ways:

-Dumbledore thought they were *both* spies, and thought Sirius had killed Peter for some internal-Death-Eater-politics reason.

-Dumbledore had no solid evidence, and/or his political influence was not enough to set Sirius free.

-Dumbledore had reasons of his own for letting Sirius go to prison -- possibly to placate Snape, possibly because he somehow knew that it would be important in the future.