Larry Dean Harmon has pleaded guilty to laundering more than $300 million while operating the Darknet-based Bitcoin (BTC) mixing service Helix. 

On Thursday, The United States Department of Justice announced that the 38-year-old pleaded guilty to laundering more than 350,000 BTC through Helix from 2014 through 2017.

Bitcoin mixers are used to help people anonymize their Bitcoin. In the past, this has helped people cover their tracks on the Bitcoin ledger since all transactions are recorded and immutable on the blockchain. As a result, mixers have become a go-to place for criminals.

Harmon admitted in court that he knew that users mixed proceeds that were generated through drug trafficking and other various illegal activities. He also wittingly partnered with darknet markets to provide the money laundering services to their users.

In addition to Helix, Harmon operated the darknet search engine Grams, founded the digital asset wallet provider DropBit, and served as the CEO of Coin Ninja — a Bitcoin media site — where Harmon advertised his Bitcoin mixer.

Harmon forfeited more than 4,400 BTC as part of the plea (worth more than $200 million at current prices). The date of Harmon’s sentencing is yet to be determined, with the Ohio native facing up to 20 years in prison plus fines.

During previous court sessions, Harmon had sought to argue that BTC was not money, so he could not be found guilty. His reasoning was that although he did indeed mix Bitcoin through his service, Bitcoin is not money, which would mean that he cannot be charged with money laundering. The presiding Judge Beryl A. Howell, however, rejected that argument.

“The term ‘money’ [...], commonly means a medium of exchange, method of payment, or store of value. Bitcoin is these things. Indeed, [Harmon] never disputes that Bitcoin is money as that term is ordinarily used, and he concedes that Bitcoin is a form of currency,” the judge ruled.

Officials long had their sights set on Helix and Harmon, with the U.S. Internal Revenue Service Cybercrimes Unit, Belize Ministry of the Attorney General, Belize National Police Department, and the U.S. Federal Bureau of Investigation having collaborated in the investigation of Harmon and Helix since at least 2014, while the mixer was in operation over the course of several years.

In February 2020, the U.S. Financial Crimes Enforcement Network arrested Harmon for operating his Bitcoin mixer without registering it as a money service business. He was fined a $60-million penalty for the charges.

Acting U.S. Attorney Channing D. Phillips emphasized the role that darknet mixers play in helping criminal actors obscure identity, stating:

“Darknet markets and the dealers who sell opioids and other illegal drugs on them are a growing scourge. They may try to hide their identities and launder millions in sales behind technologies like Helix. But the department and its law enforcement partners will shine a light on their activities.”

Related: Alleged $366M Bitcoin mixer busted after analysis of 10 years of blockchain data

Other Bitcoin mixing services have come under fire in recent years amid a flurry of U.S. regulatory action. This past April, the operator of the Bitcoin Fog mixer was arrested in Los Angeles for having laundered roughly $336 million in BTC over 10 years.