In trying to determine the sculptor of Angel, Claudia and her brother Jamie find only one clue: the bottom of the statue bears Michelangelo's signature mark.
This discovery, however, does not solve the mystery since Michelangelo did not carve all the marble blocks that bear his mark and because the mark could certainly have been counterfeited later.
The "mystery" of Angel's origin is solved when Mrs. Basil E. Frankweiler allows Jamie and Claudia access to her secret personal files. They find a piece of paper with a sonnet and Michelangelo's signature on one side. On the other side is Michelangelo's sketch of Angel.
mrs.basil e. frankenweiler lets them look at her files and they find out that it was made by micheal angelo