(Bloomberg) — Anthem Inc., the second biggest U.S. health insurer by market value, said hackers obtained data on tens of millions of current and former customers and employees in a sophisticated attack that has led to a Federal Bureau of Investigation probe.

The information compromised includes names, birthdates, Social Security numbers, street and e-mail addresses and employee data, including income, Anthem said in an e-mail. The company will notify customers who were affected and provide credit and identify-theft monitoring services for free, Chief Executive Officer Joseph Swedish said in a letter to members.

"As soon as we learned about the attack, we immediately made every effort to close the security vulnerability, contacted the FBI and began fully cooperating with their investigation," Anthem said. The Indianapolis-based company, formerly known as WellPoint, didn't provide information on how the breach occurred or when it was discovered.