The Department of Homeland Security is investigating Mt. Gox, the largest Bitcoin exchange, for violating laws on US money exchange and money transfers—and it's grabbing the exchange's money in the process.

DHS officials refused to comment on the ongoing investigation, but they did provide a copy of the warrant that was used yesterday to seize funds that Mt. Gox had in Dwolla, a money transfer service. Dwolla is a Des Moines, Iowa company that provides one of the most popular ways to move US dollars to Mt. Gox, where they can be used to buy bitcoins.

In the warrant, a special agent with Homeland Security Investigations (HSI), states that there's probable cause to believe Mt. Gox is engaging in "money transmitting" without a license, a crime punishable by a fine or up to five years in prison. The warrant goes on to demand that Dwolla hand over the keys to account number 812-649-1010, which is owned by Mt. Gox subsidiary Mutum Sigillum LLC, and held in the custody of Veridian Credit Union.

The funds in that account "are those of Mt. Gox customers that withdraw said funds from Mt. Gox and direct their transfer to Dwolla."

Homeland Security used a confidential informant, based in Maryland, to conduct the investigation. The informant simply created accounts with Dwolla and Mt. Gox, bought bitcoins, and then changed them back into dollars. Tracing that money, HSI was able to see that the money passed through a Wells Fargo account, number 7657841313, which was created by a single authorized signer: Mark Karpeles, the president and CEO of Mt. Gox. The Dwolla account shows transfers to Dwolla going back to at least December 2011, according to the warrant.

The special agent then explains what appears to be the smoking gun: Karpeles specifically denied he was going to get into the currency exchange business. The warrant reads:

As part of the account opening process, Wells Fargo required Karpeles and Mutum Sigillum LLC to complete a "Money Services Business (MSB) Accounts, Identification of an MSB Customer" form. That document was completed on May 20, 2011 and identified Mutum Sigillum LLC as a business not engaged in money services. The application asks several questions; to include, "Do you deal in or exchange currency for your customer?" and "Does your business accept funds from customers and send the funds based on customers' instructions (Money Transmitter)?" Karpeles answered these questions "no," indicating that Mutum Sigillum LLC does not deal in or exchange money, and that it does not send funds based on customer instructions. Money transmitting businesses are required by 31 USC section 5330 to register as such with FinCEN. According to FinCEN records on May 6, 2013, neither Mt. Gox nor the subsidiary, Mutum Sigillum LLC, is registered as a Money Service Business.

The agent then gives a brief description of how Mt. Gox deals in the "crypto-currency" of Bitcoin:

Mt. Gox acts as a digital currency exchange where customers open accounts and fund the respective accounts with fiat currency, which is then exchanged into crypto-currency by Mt. Gox; the crypto-currency is known as bitcoin. Fiat currency simply refers to any money that a government has declared to be legal tender. The exchange is bidirectional and allows customers to also exchange bitcoins back into fiat currency, and then withdraw those funds. The exchange of fiat currency and bitcoins incurs a floating rate fee charged by Mt. Gox and is determined by the customer's aggregate amount of funds exchanged on a monthly basis.

We've reached out to Mark Karpeles and will update the story if we hear back.