Halliday, Margot & Chief Insp. Hubbard all discover that it was Tony who set up Margot's attempted murder. They find out because Tony grabbed Swann's key and put that key in his wife's purse and the other key (his wife's key) was put back under the staircase cover right after Swann opened the door with it (Tony assumed that he was getting his wife's key from Swann's pocket, because Swann was supposed to put the key back under the staircase cover after he killed Margot, but he got Swann's own key instead). When Tony opens the door with the key that Swann put back under the staircase cover, he sees Halliday, Margot, Insp. Hubbard & Det. Williams and he knows that he has been caught.