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MIAMI (CBSMiami) – It’s considered the largest tax fraud scam in U.S. history and its unfolding across America and in our backyard.

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Posing as IRS agents on the phone, fraudsters have scammed millions of dollars with their threatening tactics which includes warn their victims to call back ‘or else’.

CBS4 Chief Investigative Reporter Michele Gillen did call back and more.

For weeks CBS 4 News went on the hunt to expose a largely invisible world of scam artists posing as the IRS. The mission: tracing clues to find fraudsters who call and leave threatening messages, the kind that terrified taxpayers like Claudine Houdaille of Miami Beach.

“The message was telling me that I should call the number immediately and get in touch within 24 hours, something like that, otherwise they will take legal action. I really got very scared,” said Houdaille.

It’s a concern that was echoed at the IRS.

“What is different about these calls is that they are aggressive and threatening,” said Michael Dubzinsky, Director of Communications for the IRS in Florida.

He met with Gillen who told him about the victims she has met with.

“I’ve seen people after getting the calls and listening to the messages, they were really shaken up,” said Gillen.

“Yes. There are a lot of people victimized here in Florida. We have had about 285 victims who have paid out more than 1.6 million dollars,” said Dubzinky.

Some taxpayers have been robo-called hundreds of times.

The recorded message, which sounds computer generated, went like this:

“‘Hi, this is agent Kimberly Grey calling from the tax fraud investigation unit of the Internal Revenue Service.”

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Most alarming was the following threat.

“The Internal Revenue Service has issued an arrest warrant against you. My direct call back number is area code 206- XXX-XXXX.”

And so CBS4’s efforts began to find out who may be at the other end of that number. First the number was cross referenced and it turned up as a listed number for a supposed tax school located in Seattle, Washington.

Every time the phone was answered, the person on the other end of the line answered, “IRS, how may I help you?”

When CBS 4 raised questions about whether the number was for what appeared to be an IRS endorsed tax school as advertised on the web, they hung up. The school’s website was then shown to Dubzinksy.

“As an IRS official I can tell that is a fake. We don’t endorse particular companies,” he said.

CBS4 then called dozens of numbers on the tax school website that listed other tax schools in cities across America. Our calls were either hung up on, their lines had been disconnected, or were told we had reached the wrong number.

We followed one advertised South Florida address and it led to a warehouse area in Doral. Signs advertised it as a box company. Employees and the property owner told us there was no tax school on the premises.

The CBS4 team called back the Seattle phone number which again answered as “Thank you for calling the IRS department.”

When Gillen asked whether this was a school, the answer was ‘no’. When she asked whether this was where an IRS payment could be made, the answer was ‘yes.’

CBS4 news continued its attempts to leave a message for the owners of the tax school as described on their website. A visit to the address of the reported school in Seattle turned out to be a mail drop and box store.

The IRS said its tough to pull back the veil of tracing phone numbers that may be tied to fraudsters with or without the knowledge of companies, schools, persons, even police departments which in some cases their numbers have illicitly appeared.

Dubzinsky said if you receive a phone call from someone claiming to be an IRS agent, do not engage in conversation with them and hang up. He said the IRS will never:

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-Call to demand immediate payment
-Require you to use one payment method
-Ask for any credit card number over the phone
-Threaten to have the police arrest you