The Social Security numbers of millions of South Carolinians, as well as credit and debit card information for hundreds of thousands, have been hacked in what the state's governor described Friday as an international cyberattack.

"This is not a good day for South Carolina," Gov. Nikki Haley told reporters.

The governor explained that a "server that warehouses all our taxpayer information was breached and taxpayer information was stolen."

The state's Department of Revenue explained in a press release that it first learned of a possible breach on October 10, after which the state contracted information security firm Mandiant to conduct an investigation.

The "hole" in the system was closed October 20. Over the next several days, state authorities determined that more than 3.6 million Social Security numbers may have been affected. So, too, were 387,000 credit card numbers - though only 16,000 of those were unencrypted.

On Friday, state officials laid out efforts to determine what happened and protect the personal information of taxpayers. While noting that not everyone had their information breached, Haley urged everyone who filed a tax return in South Carolina from 1998 through now to take advantage of credit protection services being offered by the state.

"While we now have it protected, we want to make sure that everybody understands that our state will respond with a big, large-scale plan that is somewhat unprecedented to take care of this problem," the governor said.