The fraudulent calls came suddenly. A person posing as an IRS or immigration official was on the phone, threatening Americans with arrest, deportation or other penalties if they did not immediately pay their debts with prepaid cards or wire transfers.

Thousands of people in the United States complied with the demands to pay between 2012 and 2016, the Department of Justice said.

But Justice Department officials say they have broken up what appeared to be the nation’s first large-scale, multinational operation that bilked Americans of “hundreds of millions of dollars” over four years, using call centers based in India to route the money to conspirators in the United States.

In an announcement Friday, the department said 21 people living in eight states — Illinois, Arizona, Florida, California, Alabama, Florida, New Jersey and Texas — were sentenced last week in Houston to prison for up to 20 years for their role in the scheme.

Two other conspirators in Illinois were sentenced in February to between two years to just over four years for conspiracy, and a third person in Arizona was given probation in a plea agreement, it said.

In addition, 32 contractors in India involving five call centers in Ahmadabad, a city in western India, have been indicted on wire fraud, money laundering and other conspiracy charges as part of the operation, the department said.

They have yet to be arraigned, it said.

The sentences “represent the culmination of the first-ever large scale, multi-jurisdiction prosecution targeting the India call center scam industry,” Attorney General Jeff Sessions said in the statement.

The investigation took years of work by Justice Department, Department of Homeland Security, Immigration and Customs Enforcement and Treasury Department officials, the statement said.

The IRS has repeatedly warned Americans, especially just before the April deadlines to file taxes, about scams like this one.

For example, in tips on how to avoid fraud, the IRS says it does not demand immediate payment of debts using a specific method, such as a prepaid debit card, gift card or wire transfer.

Generally, the IRS will first mail a bill to a taxpayer who owes money, allowing for questions or a formal appeal. It will not threaten to bring in the police, immigration officers or other law-enforcement officials, or revoke licenses or immigration status, the IRS says.

“Threats like these are common tactics scam artists use to trick victims into buying into their schemes,” the IRS says.

This scheme affected some of the most vulnerable members of the public, such as older Americans and immigrants, the Justice Department said.

It involved defendants who were of Indian origin living in the United States and coordinating with the call centers in India, federal officials said, citing information obtained partly through their plea agreements.

Workers at the call centers impersonated officials from the IRS or U.S. Citizenship and Immigration Services, threatening people in the United States with arrest, imprisonment, fines or deportation if they did not pay alleged debts to the government, the Justice Department said.

They chose their victims through information obtained from “data brokers” or from other sources, the department said.

The people who were duped were instructed to pay using prepaid cards or to wire money. The call centers then turned to a network of “runners” based in the United States, who typically used the cards to purchase money orders that were deposited into bank accounts.

Runners also redeemed funds from wire transfers with false names and identification documents, or exploited the funds through gift cards and Apple iTunes cards, it said. The runners would earn a fee or a percentage, it said.

“This type of fraud is sickening,” said Ryan Patrick, the U.S. attorney for the U.S. District Court for the Southern District of Texas in Houston, where last week’s sentences were imposed.

Of the defendants sentenced last week, 22 were told to pay restitution of about $8.9 million to victims who could be identified, the department said. The sentences also included judgments of more than $72.9 million.

The department named all 24 defendants, but it highlighted the role of the three men with the longest sentences.

Miteshkumar Patel, 42, of Illinois, was responsible for laundering between $9.5 and $25 million in his role overseeing a Chicago-based network of runners, the department said. He was given a 20-year sentence.

Hardik Patel, 31, of Illinois, was given more than 15 years in prison for wire fraud conspiracy as a coordinator with call centers in India, laundering between $3.5 million and $9.5 million, it said.

In Texas, Sunny Joshi, 47, was sentenced to about 12 years in prison for money laundering of between $3.5 million and $9.5 million, the statement said.