So far Bitcoin has escaped serious regulation in the US, but after the collapse of Mt. Gox, government agencies stepped up their efforts. Part of the trouble so far has been nailing down exactly what Bitcoin is. Federal Reserve chairwoman Janet Yellen has already said that it falls outside of her jurisdiction since Bitcoin is not issued by a central bank. The Commodity Futures Trading Commission is trying to determine if it could possibly be considered a commodity, while the SEC is working on whether or not it's a security. The key to its case will be whether or not MPEx represents an unlicensed securities trading platform, which would put it in violation of US law. Of course, whether or not the SEC even has any jurisdiction over the Romanian exchange or the foreign company at the heart of the investigation is another question entirely.