The Internal Revenue Service announced in May that hackers had gotten into its system, and it estimated then that 114,000 taxpayers' accounts had been compromised. But a thorough review of activity on the IRS site throughout the 2015 tax-filing season revealed that the intrusion affected nearly three times that many accounts, the agency said Monday.
In its investigation, the IRS found about 220,000 additional instances where intruders were able to clear a verification system meant to keep sensitive tax information safe, as well as about 170,000 failed attempts to get around the verification system.
That puts the agency's estimate of the total size of the breach at 330,000 accounts, and its estimate of total failed attempts to get into its system at 281,000.
The IRS said in June that it believes the attack came from Russia.
The hackers were able to use information about taxpayers that they had acquired separately to "prove" to the system that they were authorized users. They targeted the agency's "Get Transcript" service, which allows taxpayers to call up their tax history and important information.
The service required users to enter personal information about themselves to confirm their identities, suggesting that hackers had access to many pieces of information about the individuals they targeted.