The Internal Revenue Service (IRS) said on Friday that its investigation into a hack attack revealed in May last year found the cyber criminals had tried to access about 390,000 additional taxpayer accounts.
The tax agency said that 295,000 transcripts were also targeted but that in those cases, access was unsuccessful.
The agency said in May the tax return details of around 114,00 US taxpayers had been illegally accessed over the previous four months, with a further 111,000 unsuccessful attempts made.
Then in August, the IRS revealed a review had identified 220,000 further incidents where data was breached and another 170,000 suspected failed attempts to obtain taxpayers’ data.
Between February and May, cyber criminals attempted to access personal tax information via the agency’s “Get Transcript” online application which shows information from previous tax returns, the agency said in May.
The review, which took nine months, looked into incidents dating back to the launch of the Get Transcript application in January 2014 through to May 2015, and was conducted by the Treasury Inspector General for Tax Administration, according to the IRS on Friday.
The Get Transcript web app has since been offline after the incident, the agency said, adding that mailings to the affected taxpayers will commence on February the 29th.
CNN reported in May 2015 that the IRS believed the data theft originated in Russia.