Anthem, the second biggest health insurer in the USA, has revealed that the personal data of millions of current and former members was breached by hackers.
In a statement, the insurer said it "was the target of a very sophisticated external cyber attack."
The attackers gained access to Anthem's IT system and obtained personal information from our current and former members such as their names, birthdays, medical IDs/social security numbers, street addresses, email addresses and employment information, including income data.
It is the second largest health insurer in the USA with almost 40 million members.
However, the insurer said, there is no evidence that credit card or medical information, such as claims, test results or diagnostic codes were targeted or compromised.
Joseph R Swedish, president and CEO of Anthem told customers: "Once the attack was discovered, Anthem immediately made every effort to close the security vulnerability, contacted the FBI and began fully cooperating with their investigation.
"Anthem has also retained Mandiant, one of the world's leading cybersecurity firms, to evaluate our systems and identify solutions based on the evolving landscape.
"Anthem's own associates' personal information - including my own - was accessed during this security breach. We join you in your concern and frustration, and I assure you that we are working around the clock to do everything we can to further secure your data."
Anthem said it will individually notify current and former members whose information has been accessed.
It will also provide credit monitoring and identity protection services free of charge to those affected.
A dedicated website www.AnthemFacts.com was created for members to access information such as frequent questions and answers.
Swedish continued: "I want to personally apologize to each of you [customers] for what has happened, as I know you expect us to protect your information.
"We will continue to do everything in our power to make our systems and security processes better and more secure, and hope that we can earn back your trust and confidence in Anthem."