German political scientist Jürgen Falter has devoted much of his career to studying Nazi membership records and has written extensively on the rise of Adolf Hitler and his party.

He had previously looked up his own mother’s denazification records, which are kept in local state archives in Germany and typically contain post-war questionnaires taken during the allied-led process that followed World War II.

He found that she had been classified as “exonerated,” meaning she was cleared of complicity in the regime. A false statement on this questionnaire could have resulted in punishment.

So, when German newspapers earlier this year launched searchable databases allowing people to check whether their ancestors had been members of the Nazi party, Falter told CNN he was “more than surprised” to discover that his mother’s name appeared among the old party records – a secret she had apparently kept hidden even from her family.

“Given my mother’s entire character, mentality and political convictions as a liberal Catholic, it was actually inconceivable that she would have joined the NSDAP (National Socialist German Workers’ Party) in 1940 at the age of 23. But it is documented in the card index, which indicates that she was probably indeed a member,” said Falter, a senior research professor at Johannes Gutenberg University Mainz.