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IRS believes thieves who stole tax data from 100K were from Russia: AP, citing sources

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The Internal Revenue Service believes that the thieves who gained access to information for more than 100,000 taxpayers were Russian, the Associated Press reported, citing sources.

Thieves used an online service provided by the U.S. government's tax collecting agency to gain access to information from more than 100,000 taxpayers, the IRS revealed Tuesday.

Although IRS Commissioner John Koskinen declined to tell the media if the incursions originated from overseas, the AP cited two unnamed officials briefed on the matter.

Read MoreIRS says thieves stole tax info from 100,000

The information included tax returns and other tax information on file with the IRS.

The IRS said the thieves accessed a system called "Get Transcript." 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.

Koskinen said on a Tuesday conference call that the hacking was conducted in an organized manner, and that there were about 200,000 attempts to illegally access information—about half of those were successful.

"We're confident these are not amateurs, these are actually organized crime syndicates that not only we but everyone in the financial industry are dealing with," Koskinen said.

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—The Associated Press and Reuters contributed to this report.