I have riddle for you.?

There was a queen a king and a princess living in a circle castle. One Sunday the princess was murdered. The went up to the maid and said what were you doing when my daughter was killed and she said i was cleaning the furniture. The king went up to the librarian and said what were you doing when my daughter was killed and she said I was reading in the corner Than the king went up to the Butler and said what were you doing when my daughter was killed and he said I was reading the sunday paper. Who killed the daughter and how do you know?