The U.S. Secret Service detected a security breach at the S.C. Department of Revenue on Oct. 10, but it took state officials 10 days to close the attacker’s access and another six days to inform the public that 3.6 million Social Security numbers had been compromised.

The attack also exposed 387,000 credit and debit card numbers. The stolen data included other information people file with their tax returns such as names and addresses. Businesses’ taxpayer identification numbers also potentially have been comprised in the attack that is being described as one of the nation’s largest against a state agency.

The attack affects tax returns as far back as 1998, the Revenue Department said. But not all of the department’s data – so not every taxpayer – was affected, it said.