CHARLESTON, S.C. — About 3.6 million tax returns from as far back as 1998 were hacked in South Carolina and specialists said Wednesday it may be the largest cyberattack against a state tax department in the nation’s history.

State and federal officials are investigating the hacking they say may have started in August and was discovered last month. They say the vulnerability in the system has been fixed. The 3.6 million tax returns filed since 1998 included millions of Social Security numbers and about 387,000 credit and debit card numbers that were also exposed, 6,000 of those unencrypted. Tax information from businesses across the state may also have been accessed.