Clue by Clue: The Wolf in Lamb's Clothing Solution
Ms. Williams used the frozen leg of lamb to kill Ms. Salem. A frozen leg of lamb is very large and also very heavy and very hard. Mrs. Williams cooked the lamb and fed it to her family, so the police never thought of looking at it. She clearly invented the story of the man in black with the hammer. No one else saw him and a hammer was too small.