Synthesized answer
The author provides the following evidence:
Herr Karl, the head waiter of the Café Burg, is found dead at the foot of the Maria Theresa monument [1]. While it is initially assumed he slipped on ice and hit his head, a witness has come forward with a different account [1]. Additionally, private enquiries agent Karl Werthen is hired to investigate this death [1]. Werthen is also commissioned to find a missing letter from the Emperor to his mistress [1]. The Emperor is concerned about this letter falling into the wrong hands, and its contents are described as potentially incendiary [1]. As Werthen and his colleague, Dr. Hanns Gross, investigate, more bodies are found, suggesting a connection between the two cases [1].
The passages do not explicitly detail the specific evidence gathered during the investigations, such as witness testimonies beyond the initial one, forensic findings, or concrete clues pointing to the perpetrator or the content of the letter. However, it establishes that a witness has come forward and that there is a growing body count which suggests a connection between the death of Herr Karl and the missing letter [1].
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: The Third place by J. Sydney Jones Description: March, 1902. When Herr Karl, head waiter of the Café Burg, is found dead at the foot of the Maria Theresa monument, it is assumed he slipped on the ice and hit his head. However, a witness has come forward who says otherwise, and private enquiries agent Karl Werthen is hired to investigate. At the same time, Werthen is commissioned to locate a missing letter from the Emperor to his mistress. Franz Josef is desperate for the letter not to fall into the wrong hands - but what incendiary information does it contain? As Werthen and his…