With stiff sentences for 21 conspirators last week in the United States and a round of indictments in India, the Justice Department says it has broken up what appeared to be the nation’s first large-scale, multinational telephone fraud operation.

Over four years, more than 15,000 victims in the United States lost “hundreds of millions” of dollars to the sophisticated scam, and more than 50,000 individuals had their personal information misused, the department said Friday. The money was routed through call centers in India back to the ringleaders in eight states.

The fraudulent calls came suddenly and frequently while the scam was active from 2012 to 2016, according to court documents. A person posing as an Internal Revenue Service or immigration official was on the phone, threatening arrest, deportation or other penalties if the victims did not immediately pay their debts with prepaid cards or wire transfers.

The calls targeted the most vulnerable Americans, including immigrants and older people.

[Worried about getting scammed? Here’s how to protect yourself.]