Authorities said Tuesday that they have arrested five people in Miami who are accused of tricking 1,500 victims into paying them a total of $2 million in “taxes” they didn’t owe, part of a far-flung scam that so far has netted criminals $36.5 million.

The suspects, all Cuban nationals who were arrested Monday, were charged with wire fraud and conspiracy to commit wire fraud. They falsely impersonated IRS agents, Treasury Department authorities said, demanding money from their victims and telling them they would be arrested if they didn’t wire payments immediately.

In recent months, the scammers demanded that their victims pay them using  iTunes gift cards.

“The scammers are relentless and so are we,” said J. Russell George, treasury inspector general for tax administration, whose office has been investigating the phone scam since August 2013 and made the arrests.

Officials cautioned that the IRS never calls a taxpayer to demand immediate payment.

Deputy Inspector General Tim Camus called the scam the “largest and most pervasive” the IRS has faced in 30 years. In all, about 6,400 victims have handed over $36.5 million to criminals, each paying  $5,700 on average.

The scammers were tracked down following a tip that came into the Senate Aging Committee’s fraud hotline last year.

Those arrested were: Jennifer Valerino Nunez, Dennis Delgado Caballero, Arnoldo Perez Mirabal, Yaritza Espinosa Diaz and Roberto Fontanella Caballero. The criminal complaints were referred to offices of the U.S. attorney in Minnesota, Texas and Arkansas, where the victims erroneously sent payments.

The scammers told victims that they had their tax returns and needed to verify information to process them. They asked for personal information before demanding that they wire money to pay off their debt.

Camus called the scheme an “old fashioned crime” committed with modern technology. The criminals were able to mask the telephone numbers they were calling from with Voiceover features. The calls were made from locations outside the United States, authorities said.