The U.S. Secret Service asked Tether to freeze the tokens during a “months-long investigative effort” by Tether and the crypto exchange OKX, Tether said in a blog post, without giving details about the scope or timescale of the investigation.

The Secret Service did not immediately respond to a Reuters request for comment.

Tether and OKX collaborated with the U.S. Department of Justice (DOJ) on the investigation, Tether said without elaborating. A spokesperson for the DOJ waited to respond to requests for comment.

The crypto tokens were “linked to an international human trafficking syndicate in Southeast Asia responsible for a global “pig butchering” romance scam,” Tether said.

The term “pig butchering” typically refers to instances when a scammer builds trust with their victims over social media, messaging, and dating apps, then pressures them to invest in bogus crypto or online trading schemes.

Tether did not give further details about the group or how they used the cryptocurrency.

Tether said it was the largest-ever freeze of its token.

Hundreds of thousands of people are being trafficked by criminal gangs and forced to work in scam centers and other illegal online operations in Southeast Asia, the United Nations said in a report in August.

Tether is a so-called stablecoin, pegged to the U.S. dollar. There are $87.9 billion tether tokens in circulation, making it the third biggest cryptocurrency after bitcoin and ether, according to CoinGecko data.

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