Now those foreign assets may be vulnerable to sanctions. Persons under sanctions are barred from traveling to the U.S., and their assets are blocked. U.S. companies and individuals are prohibited from doing business with them.

EU Sanctions

The European Union has sanctioned 83 people in connection with the Ukraine crisis. It added two companies, including oil and natural gas producer Chernomorneftegaz, to its blacklist on May 12. The EU leaves enforcement of sanctions to its 28 member countries. Latvian authorities said in March that they froze assets of a sanctioned Ukrainian citizen whom they declined to identify.

Switzerland’s federal prosecutor has frozen 170 million Swiss francs ($193.7 million) in bank accounts belonging to ousted Ukrainian president Viktor Yanukovych and people close to him, according to a Swiss government spokesman. Switzerland is not part of the EU. Yanukovych fled to Russia in February after massive street protests against him.

Tobon declined to say whether any blacklisted Russians own property in the U.S., citing the ongoing investigation. There’s a “good possibility” that the sanctions list includes owners of U.S. assets, though the links may be difficult to prove and could be challenged in court, said Jack Blum, a former U.S. Senate investigator and expert on money laundering and offshore havens.

London, Manhattan

Russian billionaires have acquired real estate in world- class destinations such as London, Miami, northern California and Manhattan. In some recent transactions, the buyers aren’t publicly known. In 2012, for example, an unidentified Russian purchased a bayfront Miami mansion with $47 million in cash, according to the Miami Herald.

The sanctions over the Ukraine crisis differ from recent programs targeting Iran, Syria, North Korea, and Libya because Russia has close ties to U.S. and European economies, Serena Moe, consulting counsel for Wiley Rein LLP and former deputy chief counsel for the Office of Foreign Assets Control at Treasury.

“The targets here are incredibly wealthy, it’s a big economy and there’s a lot of interplay,” Moe said.

Edward Mermelstein, a New York attorney who represents international business clients, said investigators are unlikely to find anything because Russian magnates tend to be prepared in advance for potential risks to their investments.