Those numbers are staggering.
In the last two years, more than 400,000 people have filed complaints with the U.S. Treasury Department about these IRS imposters. More than 3,000 people have been defrauded out of a combined total of more than $15 million, McCaskill told NBC News.
The calls keep going out - more than 10,000 a week, according to the Treasury Inspector General for Tax Administration (TIGTA).
Seniors are often targeted for these calls, because they're home, they'll answer the phone and they often have the money to pay the scammers.
"But this is not limited to seniors. Anyone can be a victim," noted John Breyault, who runs the National Consumers League's Fraud.org website. "They're very good at what they do - steal people's money."
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Why is it working?
The IRS imposter scam is not an especially sophisticated con, but people keep falling for it. Doug Shadel, a fraud prevention expert with AARP's Fraud Watch Network, says it works because the crooks use fear tactics to get the response they want.
"They're threatening to arrest you, if you don't pay on the spot. Even if you don't feel that you owe the money, many will pay out of fear in order to avoid the possibility that something bad is going to happen," he explained.
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Shadel got one of these calls a few weeks ago. He provided NBC News with a transcript of the message the IRS imposters left him.
Hi, this is Officer John White. We are calling you from criminal investigation department of IRS. The reason you are getting a phone call from the department is to inform you that IRS has got the arrest warrant out for you and your physical address is under federal investigation. So call me back on my department number to get the detailed information about your case. My call back number is 254-449-XXXX. I repeat, it is 254-449-XXXX. It's very important to hear from you today."
Shadel called the number to see what the scammers would say. He was told the he owed $2,700 in back taxes, that his call was being monitored and recorded by Homeland Security, and that if he didn't agree to pay immediately, a warrant for his arrest would be signed.
"If I had not known this was a scam, I would have been terrified," Shadel said.