The IRS is paying out billions of dollars in fraudulent tax refunds to identity thieves; a problem that the tax service’s inspector general told CNBC is a “growing problem” involving numbers that are increasing “exponentially.”

In a new report to be issued Thursday, the inspector general for the IRS says that tax thieves are stealing the identities of taxpayers and then filing bogus returns on their behalf and collecting fraudulent refunds as a result.

The inspector general estimates that the IRS could issue as much as $21 billion in fraudulent tax refunds over the next five years.

The scam is so rampant that thieves are apparently sending in false returns in bulk without even bothering to change the mailing address on the returns. The inspector general said it found one residential address in Lansing, Michigan that was the source of an astonishing 2,137 tax returns, and to which the IRS directed more than $3.3 million in potentially fraudulent refunds.