There was a book written about 15 to 20 years ago that explains how this happened with the SS agent being upset when the limo accelerated. Explains why the secrecy for the last 50 years.
What a coincidence! The author of the article just happens to be the same author of the book you mentioned.........
This article is a great read and explains how and why he decided to contact Donahue and ultimately write the book .