After Claudia and Jamie read the letter from the museum, Claudia is so disappointed that all she can do is cry.

The letter from the museum is very polite.  In it, the museum acknowledges that they have known about the clue the children have sent for a long time, and that despite this, they still aren't sure whether or not Michelangelo was indeed the creator of Angel.  Claudia and Jamie had so hoped that their clue would provide the necessary evidence that Michelangelo was the statue's sculptor, and are very much let down by the news that it does not.  Jamie, while disappointed, accepts the reply pragmatically and is ready to go back home, but for Claudia, much more than a simple rejection is involved.  Solving the mystery of the statue would have allowed her to accomplish something that would have let her return home different, and the fact that she has not been able to do what she set out to do leaves her completely frustrated and unfulfilled.  Claudia feels that she simply cannot go home until she has made a real discovery, and decides that she and Jamie must make one more stop before returning.  As a last resort, they will go to visit Mrs. Basil E. Frankweiler herself (Chapter 8).