Thousands of Americans nationwide have been targeted since August by a phone scam in which fraudsters claim to be from the U.S. Internal Revenue Service and demand money for unpaid taxes, said the IRS' watchdog on Thursday.
The Treasury Inspector General for Tax Administration (TIGTA) said it has received more than 20,000 complaints from people, including recent immigrants, about the scam.
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Thousands of victims have collectively paid more than $1 million to the scammers, TIGTA said.
"This is the largest scam of its kind that we have ever seen," said J. Russell George, the head of TIGTA in a statement.
(Read more: Big data used to catch fraudulent tax returns)