Gov. Nikki Haley says South Carolina used the same standards as banks and other private institutions when it decided not to encrypt Social Security numbers and other information on a database of state tax returns.
Authorities say an international hacker was able to get into the database months ago. The hacker may have stolen up to 3.6 million returns from as far back as 1998.
Investigators are still trying to determine how much information was stolen. Tax returns can include names, address, Social Security numbers and bank account information.
The state is offering free credit monitoring to anyone affected. Haley said Monday that more than 450,000 people have swamped a phone bank and more than 150,000 people have signed up for the monitoring since it started Friday.