WITH numerous governments already announcing probes into the “Panama papers” and others preparing to do so, Mossack Fonseca, the law firm from which the hoard of documents about offshore companies was leaked, will be receiving lots of inquiries in the coming months. Until now, getting information on clients of law firms in Panama has been about as common as ice-skating on the Canal. But sleuths may soon find it a lot easier, thanks to a court ruling in, of all places, Las Vegas.

In 2014 Elliott, a fund that owned debt on which Argentina had defaulted, sued in Nevada to compel Mossack’s local affiliate to provide information on shell companies, in the hope of discovering Argentine assets to seize. The affiliate, MF Nevada, claimed—implausibly—that it was independent of Mossack. In a series of legal skirmishes, Elliott and its lawyers from Dechert LLP established numerous links between the two: for instance, that the employment contract of MF’s sole employee was signed by Mossack partners. E-mails among the Panama papers are said to show that the firm tried to hide evidence of its control over MF Nevada, and wanted its local representative to lie about the relationship. (Mossack has denied this.) One manager reportedly worried that “it could easily become clear that we are hiding something.”

A judge in Las Vegas ruled in March 2015 that Mossack and MF Nevada were one and the same. That put a crack in the wall of secrecy around American shell companies. But its full significance is only now becoming apparent: it means that, under an American law about assisting with foreign legal proceedings, any investigator anywhere in the world can subpoena Mossack, through the Nevada subsidiary, for information that could be relevant to cases in any country. (Mossack plans to appeal.)

In short, the little-noticed ruling is the key that unlocks the door for investigators everywhere. Faced with the power of American subpoenas, Mossack’s head office will find it much harder to stonewall foreign requests for information. Ignoring them could mean being found in contempt of court. That would leave it open to penalties designed to compel it to comply, including asset seizures, in other countries where it operates.

The firm could, of course, slam the door shut by closing its American business. But it is contractually obliged to provide services for thousands of companies there, and cannot disentangle itself from them overnight. Still, those looking to take advantage of the trail Elliott has blazed shouldn’t hang around.