After speaking with her friend in King's Landing about Bran's murder attempt, why will Lady Catelyn conclude that Tyrion Lannister tried to kill Bran?

Lady Catelyn will conclude that Tyrion tried to kill Bran because he was at Winterfell when Bran fell from the window, and Tyrion was the one who found Bran's body. Tyrion owns the dagger that was used to try to kill Bran in his room.