The IRS says criminals gained access to the accounts of more than 100,000 taxpayers through its online service Get Transcript. The data stolen included taxpayers' Social Security information, when they were born and their street addresses.

At a news conference, IRS Commissioner John Koskinen said criminals made about 200,000 attempts to access tax information; 100,000 of those attempts, made from February to mid-May, were successful.

The IRS became aware of the breach when workers noticed a larger than usual number of people seeking transcripts through the online service. The thieves used the data to file fake tax returns. Koskinen said less than $50 million had been successfully claimed from the agency.

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

The IRS, in a statement, said the criminals "gained sufficient information from an outside source before trying to access the IRS site, which allowed them to clear a multi-step authentication process, including several personal verification questions that typically are only known by the taxpayer."

The agency said the Treasury inspector general for tax administration and the IRS's Criminal Investigation unit are reviewing the breach; the Get Transcript application has been shut down temporarily.

"The IRS will provide free credit monitoring services for the approximately 100,000 taxpayers whose accounts were accessed," the IRS said. "In total, the IRS has identified 200,000 total attempts to access data and will be notifying all of these taxpayers about the incident."

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