Mike Caldwell, the man who designed a set of collectible coins with Bitcoin keys hidden inside, has shut down his business after receiving a letter from the government.

The Financial Crimes Enforcement Network (FinCEN), a branch of the Treasury Department, informed him last month that minting physical bitcoins qualifies him as a money transmitter business, which means he needs to register at the federal level and probably get state licenses too.

Caldwell hasn't decided what to do yet. If he wants to continue doing business without fear of breaking the still-ambiguous law, he'd have to spend up to millions of dollars getting licensed in all 47 states that regulate money transmission.

Caldwell hasn't decided what to do yet

For now, Caldwell has shuttered his business, Casascius Coins, until further notice. As regulators crack down on the largely shadowy virtual currency, smaller players are being driven out in favor of venture-backed companies.