In another example, a U.S. postal inspector based in Tampa, Florida, uncovered several tax refund schemes where funds were deposited into a debit-card account.

"According to the postal inspector, the successful schemes involved identity thieves using the SSNs (Social Security numbers) of deceased people and individuals who receive public assistance," the audit said.

The IRS said it detected 938,664 fake tax returns during the 2011 processing year, which would have cost $6.5 billion. While TIGTA said the figure was "substantial," it believes the IRS doesn't know how many identity thieves are filing bogus returns and how much money is lost.

TIGTA said it studied identity-theft characteristics and found 1.5 million fraudulent tax returns that were not detected by the IRS, which cost U.S. government coffers more than $5.2 billion.