Data security experts said the breach could allow crooks to take over bank accounts, file for bogus tax refunds or obtain fraudulent loans, The State of Columbia, S.C. reported Tuesday.
"This is about the worst you can get," said Avivah Litan, an identity theft analyst with the information technology research group Gartner.
Rick Holland of Forrester Research said if he were a resident of South Carolina, he'd be "pretty concerned."
The South Carolina Senate Finance Committee scheduled a hearing Tuesday to question the director of the Revenue Department about the security breach that originated from an overseas location.
Gov. Nikki Haley said it is not known if the hackers obtained more than unencrypted Social Security numbers.
"To tell you now would be guessing," Haley said Monday.
The state has hired Experian, a California-based firm, to provide theft protection for taxpayers.