WASHINGTON — Federal authorities unsealed an indictment Thursday, charging 56 people in a vast scheme in which suspects posed as Internal Revenue Service agents and immigration authorities to siphon more than $300 million from thousands of unwitting victims to clear fictitious deportation warrants and phony tax debts.

The scheme, which employed a network of telephone call centers based in India, relied on personal information obtained from data brokers to target at least 15,000 victims with threats of fines, deportation or imprisonment if they did not pay the demanded fees.

In the thousands of cases where victims did agree to settle the fictitious accounts, the money allegedly was laundered through groups of U.S. co-conspirators using wire transfers and debit cards.

Twenty of the 24 U.S. suspects had been arrested, officials said Thursday. Thirty-two suspects were believed to be living abroad. The court documents also outlined charges against five call center operations.

"This is a transnational problem, and demonstrates that modern criminals target Americans both from inside our borders and from abroad,'' Assistant Attorney General Leslie Caldwell said of the alleged activity uncovered during a three-year investigation.

The co-conspirators, according to federal authorities, used so-called “hawala transfers” in which money is transferred internationally outside of the formal banking system to direct extorted funds to accounts belonging to U.S.-based individuals.

According to the indictment, the conspirators allegedly kept a percentage of the proceeds for themselves for taking part in the transfer.

In the case of a San Diego victim, prosecutors allege that a call center extorted $12,300 from the 85-year-old woman after threatening her with arrest if she did not settle phony tax violations. The same day that the payment was made, a U.S.-based suspect allegedly loaded a debit card in the amount of the payment to purchase money orders in Frisco, Texas.

Another California victim lost $136,000 to suspects posing as IRS agents demanding payment for fictitious tax charges. The suspects, according to court documents, contacted the victim multiple times during a period of 20 days. The money was then allegedly transferred to to multiple debit cards.

In some cases, prosecutors allege, the conspirators requested "good-faith deposits'' from victims in exchange for the promise of phony grants and loans.

"To potential victims, our message today is simple,'' said Peter Edge, executive associate director with Immigration and Customs Enforcement. "U.S. government agencies do not make these types of calls. And if you receive one, contact law enforcement ... before you make a payment.''