Sirius went after Peter as soon as he saw the house. He knew immediately what Peter had done. Peter blew up a bloody street, killed a ton of Muggled, cut of his finger and turned himself into a rat, leaving Sirius to take the blame. He was hauled off to Azkaban straight away, where he rotted in a cell for 12 bloody years. He tried to tell him he was innocent, but no one would listen to him.