The father of billionaire brothers Charles and David Koch helped construct a major oil refinery in Nazi Germany, a new book reports.

Adolf Hitler personally approved the refinery, according to “Dark Money” by Jane Mayer, which looks at some rich donors to what the book terms in its subtitle the “radical right” of today while focusing on the Koch family.

Fred Koch and American Nazi sympathizer William Rhodes Davis were partners in the refinery, the third largest in the Third Reich and a critical industrial cog in Hitler’s war machine, according to The New York Times.

Koch Industry company officials did not participate in the book. The refinery is not mentioned in an online history published by Koch Industries, according to the Times.

The Koch brothers have given more than $100 million to conservative and libertarian policy and advocacy groups in the United States, Mayer reported in the New Yorker, where she works as a staff writer, in 2010.