When reporters Will Fitzgibbon and Debbie Cenziper began digging into the Pandora Papers — nearly 12 million confidential files leaked to the International Consortium of Investigative Journalists whose contents would go on to reverberate across the world — they noticed something curious. In the trove of documents, which expose a secretive financial system that shields the deals and assets of some of the world’s richest and most powerful people, one U.S. state, South Dakota, stood out. “We early on found a document in the leaked records that showed that there were a number of trusts established in South Dakota, and...