The IRS said cyberattacks on the agency may have been seven times larger than it initially reported.
A new government review found hackers potentially accessed more than 700,000 taxpayer accounts starting last February.
The IRS first reported the attack in May, saying only about 100,000 accounts had been compromised.
The agency said the hackers may have stolen social security numbers and birth dates.
The IRS said it will send mailings to affected taxpayers starting Monday.
(Copyright (c) 2016 Sunbeam Television. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)