When Patrick Murck walked into a small, drab conference room at the U.S. attorney's office in Manhattan last month, the freewheeling world of the virtual currency bitcoin got uncomfortably real.

The 38-year-old Mr. Murck is general counsel of the Bitcoin Foundation, a trade group that promotes bitcoin. So he wasn't surprised when the group got a subpoena from federal prosecutors who wanted to know all about Mt. Gox, the trading exchange that collapsed after announcing it lost roughly $500 million of bitcoins, mostly owed...