Quote from "The Mind of Adolf Hitler":

“There is a great deal of confusion in studying Hitler’s family tree. Adolf’s father, Alois Hitler, was the illegitimate son of Maria Anna Schicklgruber. It was generally supposed that the father of Alois Hitler was Johann Georg Hiedler…. Alois, however, was not legitimized, and he bore his mother’s name until he was forty years of age when he changed it to Hitler.

A peculiar series of events, prior to Hitler’s birth, furnishes plenty of food for speculation.

There are some people who seriously doubt that Johann Georg Hiedler was the father of Alois. Thyssen and Koehler, for example, claim that Chancellor Dolfuss (the Chancellor of Austria) had ordered the Austrian police to conduct a thorough investigation into the Hitler family. As a result of this investigation a secret document was prepared that proved Maria Anna Shicklgruber was living in Vienna at the time she conceived.

At that time she was employed as a servant in the home of Baron Rothschild. As soon as the family discovered her pregnancy she was sent back to her home in Spital where Alois was born.”