Some 65,000 Puerto Ricans left their bankrupt U.S. island commonwealth last year. A group of private bankers are moving the other way. They’re increasingly opening offshore banks known as International Financial Entities, which were created by a Puerto Rican law in 2012. There are 44 IFEs now, with 18 opening in the past year, according to data compiled by the U.S. territory’s financial regulator. “Just in the last six months, we’ve probably closed seven deals for international banks,” says Ryan Christiansen, president of Christiansen Commercial Real Estate, a brokerage based in Puerto Rico that leases office space. Tax experts attribute at least part of the influx to a little-known loophole made possible by the IFE structure. It lets non-U.S. account holders put money in Puerto Rico anonymously and potentially avoid taxes at home even as they benefit from the stability and safety of the U.S. That’s become increasingly attractive because of a new global financial-disclosure system ta...