WASHINGTON — (CNN) The IRS believes that a major cyber breach that allowed criminals to steal the tax returns of more than 100,000 people originated in Russia, two sources briefed on the data theft tell CNN.

On Tuesday, the Internal Revenue Service announced that organized crime syndicates used personal data obtained from elsewhere to access tax information, which they then used to file $50 million in fraudulent returns.

The IRS said the agency’s Criminal Investigation Unit and the Treasury Inspector General for Tax Administration are investigating. The agency also alerted the Homeland Security Department of the breach, a DHS official confirmed.

An IRS spokeswoman said the agency does not discuss ongoing investigations.

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