(Newser) – The fallout from the massive Panama Papers leak has mostly been overseas, where Iceland Prime Minister Sigmundur David Gunnlaugsson has resigned and Britain's Prime Minister David Cameron is facing tough questions over his father's use of offshore shell companies. But US authorities are still keeping a close eye on developments: The leak has exposed hundreds of American citizens, and thousands of US companies, with ties to law firm Mossack Fonseca's offshore network. More:

It is difficult to tell exactly how many Americans are involved, but the leaked data includes at least 200 scanned US passports, McClatchy reports. Many of them appear to be American retirees using shell companies to buy property in Costa Rica and Panama. The data is also believed to include many American dual nationals using their foreign passports for financial dealings.

McClatchy also reports that four Americans accused or convicted of serious financial crimes were able to use Mossack Fonseca to set up offshore companies. They include Benjamin Wey, president of New York Global Group. He was indicted last year on securities fraud charges and is accused of scamming tens of millions from shareholders.

Incorporation records show that Mossack Fonseca has set up more than 1,000 companies in the US since 2001, but it has been named in only a handful of court cases, mostly involving attempts to track money believed to have been hidden in offshore tax havens, USA Today reports.

Compared to other countries, there are relatively few Americans named in the leak, which analysts say could be because loose secrecy laws have made it very easy for Americans to set up shell companies in Wyoming, Nevada, or Delaware instead of going overseas. "If a company in the US can do the exact thing for you as this company in Panama, then you might as well do it right here in the US," a spokeswoman for the US Public Interest Research Group tells NBC News.

The New Yorker notes that Mossack Fonseca may have deliberately avoided doing a lot of business in the US to avoid attracting the attention of regulators—and because a 2010 deal between the US and Panama requires the latter country to provide American authorities with information on the ownership of companies, among other things.

(Read more Panama Papers stories.)