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The Internal Revenue Service revealed Monday that criminals potentially accessed tax information for a total of 338,000 taxpayers — triple the amount feared when the breach was first disclosed in May.

The IRS had originally said that hackers potentially accessed information for 114,000 taxpayers via an online system. The agency did a further review and issued a statement Monday adding 224,000 more victims to the list of those whose information was potentially accessed.

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"As part of the IRS’s continued efforts to protect taxpayer data, the IRS conducted a deeper analysis over a wider time period covering the 2015 filing season, analyzing more than 23 million uses of the Get Transcript system," the IRS said in its statement.

Related: Can the Government Protect Itself From Hackers?

The attackers were able to look at taxpayers' filings because they "confirmed" their identities by entering personal data like Social Security numbers, dates of birth and street addresses -- which experts say are routinely sold online between criminals for just a few dollars.The IRS believes the information was stolen as part of a scheme to claim fraudulent tax refunds in the future.

The 338,000 figure doesn't include the number of failed fraudulent attempts to access taxpayers' information. In May, the IRS said the attackers tried but failed to access another 111,000 accounts. On Monday,the agency added 170,000 more attempts to that figure. That brings the number of failed attempts to 281,000.

Related: IRS Breach Puts Spotlight on the Internet's 'Costco of Cybercrime'

The IRS said it is "moving aggressively to protect taxpayers whose account information may have been accessed. As it did in May, the agency will begin mailing letters to the new batch of affected or potentially affected taxpayers over the next few days.