U.S. investigators believe the identity thieves who stole personal information of more than 100,000 taxpayers from an Internal Revenue Service (IRS) website are part of a criminal operation based in Russia.
The information was stolen as part of an elaborate scheme to claim fraudulent tax refunds, IRS Commissioner John Koskinen said.
Officials who spoke on condition of anonymity said computer data traces the criminals to Russia.
The schemers stole taxpayer information through an IRS website called Get Transcript, where taxpayers can get copies of their past tax filings.
The case highlights the global reach of cybercriminals. It's not the first time the IRS has been targeted by identity thieves based overseas.
In 2012, the IRS sent 655 tax refunds to a single address in Lithuania.
The IRS has since added safeguards to prevent similar schemes, but the Russian thieves were able to get around them.
Koshigen is scheduled to testify about the latest fraud before the Senate Finance Committee on June 2.