Health insurer Anthem says hackers infiltrated its computer network and accessed a swathe of personal information about current and former customers including their incomes and street addresses.
The company says in a statement that the cyberattack was "very sophisticated."
It says credit card information wasn't compromised and so far it has not found evidence that medical information such as insurance claims and test results was targeted or obtained.
Anthem says the FBI is investigating and it has hired Internet security company Mandiant to improve its network defenses.
Information stolen included names, birth dates, social security numbers, street addresses, email addresses and employment details including income.