WASHINGTON – The Internal Revenue Service may have delivered more than $5 billion in refund checks to identity thieves who filed fraudulent tax returns for 2011, Treasury Department investigators said Thursday. They estimate another $21 billion could make its way to ID thieves' pockets over the next five years.

The IRS is detecting far fewer fraudulent tax refund claims than actually occur, according to a government audit that warned the widespread problem could undermine public trust in the U.S. tax system. Although the IRS detected about 940,000 fraudulent returns for last year claiming $6.5 billion in refunds, there were potentially another 1.5 million undetected cases of thieves seeking refunds after assuming the identity of a dead person, child or someone else who normally wouldn't file a tax return.

In one example, investigators found a single address in Lansing, Mich., that was used to file 2,137 separate tax returns. The IRS issued more than $3.3 million in refunds to that address. Three addresses in Florida, the epicenter of the identity theft crisis, filed more than 500 returns totaling more than $1 million in refunds for each address.