If you’re thinking of recruiting members for a Ponzi scheme in South Korea, avoid using terms like “recruiting members” and “Ponzi” too much. Two alleged crypto fraudsters known only as Lee and Bae were arrested last Thursday after investigators were alerted by an AI program trained to pick up terms like those. The two men were thought to have profited $18.7 million from the scheme, Korea JoongAng Daily reports.

The scheme had affected around 56,000 people over at least six months, according to the Seoul Special Judicial Police Bureau for Public Safety. Most of the victims seem to have been targeted because of their limited understanding of cryptocurrency. Investigators said the majority were between 60 and 80 years old.

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The scheme involved an exchange where members could buy a cryptocurrency called M-coin. Lee and Bae allegedly promised that the coin would soon multiply in value—although there was nothing special about it. They expanded by rewarding members for recruiting other members.

The scheme generated a huge profit because membership fees far outweighed rewards handed out. Participants would only be given about $52 for bringing in a new member, while regular membership cost $287 and premium cost $863.

The illusion of legitimacy was partly reinforced by the exchange’s location in Gangnam, a fashionable area of Seoul.

Life is made easy for fraudsters by relatively poor public understanding of cryptocurrencies. A 400 percent rise in scammed and stolen cryptocurrency brought last year’s total to around $1.7 billion, according to U.S. cyber security firm CipherTrace. South Korea and Japan were hotspots, with 58 percent of that between them.

“We have seen in 2018 a lot more exit scams where companies disappear and steal people’s money. There’s huge increase in that,” Dave Jevans, chief executive of CipherTrace, told Reuters in January.