The theft of tax information from a South Carolina computer system appears to have been the largest cyberattack ever on a state government and has put other states on high alert, computer security experts say.

The state announced late last month that an international hacker had stolen 3.6 million Social Security numbers and 387,000 credit and debit card numbers. Now tax departments across the country are inspecting their own security systems.

“When one employee’s laptop gets stolen, it’s a big deal,” said Verenda Smith, the deputy director of the National Federation of Tax Administrators. “So you can imagine the reverberations when this news came out.”

Since 2005, at least 11 state tax agencies have faced security breaches, according to the Privacy Rights Clearinghouse, a consumer rights group. But most were caused by internal accidents, not attacks, and none were on this scale.