The scam began with a message, then a friendly exchange. A stranger directed the victim to a cryptocurrency investment site that appeared professional — slick design, charts, even customer support. The first deposit showed a modest profit. So did the next. Encouraged, the victim sent more, even borrowing money to keep up. Then, without warning, the platform stopped responding. The account balance disappeared.“That’s how they do it,” Jamie Lam, an investigative analyst with the US Secret Service,...