The IRS is increasing the number of victims from the massive computer breach first revealed by the agency back in May.

An additional 220,000 potential victims had information stolen from an IRS website as part of a sophisticated scheme to use stolen identities to claim fraudulent tax refunds, the IRS said Monday. The revelation more than doubles the total number of potential victims, to 334,000.

Fox News explains, "The thieves accessed a system called "Get Transcript," where taxpayers can get tax returns and other filings from previous years. In order to access the information, the thieves cleared a security screen that required knowledge about the taxpayer, including Social Security number, date of birth, tax filing status and street address, the IRS said."

The IRS says that the thieves were accessing the website to get more information about taxpayers so they could file for fraudulent refunds.

"As it did in May, the IRS is moving aggressively to protect taxpayers whose account information may have been accessed," the IRS said in a statement. "The IRS will begin mailing letters in the next few days to about 220,000 taxpayers where there were instances of possible or potential access to `Get Transcript' taxpayer account information."

The thieves used information from 660,000 different people in their attempt to access the IRS database. "On Monday, the IRS did not identify a potential source of the crime. But in May, officials said IRS investigators believe the identity thieves are part of a sophisticated criminal operation based in Russia."