Last Sunday, the Washington, D.C.-based International Consortium of Investigative Journalists (ICIJ) published a massive leak of some 11.5 million documents covering nearly four decades that showed how world leaders, politicians and businesses hide and launder their money, evade taxes and finance arms and drug deals. The source of the leak is a little-known but powerful law firm in Panama called Mossack Fonseca, which the ICIJ describes as one of the top creators of shell companies and corporate structures that can be used to hide ownership of assets.

The “Panama Papers,” as the leaked documents are called, offer a rare opportunity to regulators in the U.K., the U.S.…