WASHINGTON, July 29 -- When American investors tried to withdraw their money from K6 Investing, the answer was always the same: the account was frozen. When they pressed further, the response shifted. Operators threatened victims with legal action. Some were told the only way to unlock their balances was to send additional money through the same platform that was already refusing their withdrawal requests. For many, the effort to recover what they had lost created new losses.

The platforms they trusted, K6 Investing, Neotron Holding LTD and Goldex Technology, were not brokerages. They were digital interfaces engineered to display false account performance, funnel money out of US bank accounts, and then hold those apparent balances hostag...