The IRS said a hack of one of its computer databases in May was more extensive than previously believed.
The IRS said in late May that tax return information of about 114,000 U.S. taxpayers had been illegally breached by cybercriminals over the prior four months, while another 111,000 attempts were unsuccessful.
A new review has identified 220,000 additional incidents, according to the IRS.
As a result of the breach, 15,000 fraudulent returns were processed in the 2015 tax filing season, which likely resulted in refunds of less than $50 million, according to the agency. The compromised "Get Transcript" online application was consequently shut down after the attacks.
"The IRS believes some of this information may have been gathered for potentially filing fraudulent tax returns during the upcoming 2016 filing season," the agency said in a statement.
It added that it will begin mailing letters in the next few days to the taxpayers whose accounts might have been accessed by hackers.