Anthem had 37.5 million medical members as of the end of December.

"This attack is another reminder of the persistent threats we face, and the need for Congress to take aggressive action to remove legal barriers for sharing cyber threat information," US Rep. Michael McCaul, a Republican from Texas and chairman of the Committee on Homeland Security, said in a statement late Wednesday.

The FBI had warned last August that healthcare industry companies were being targeted by hackers, publicizing the issue following an attack on US hospital group Community Health Systems Inc that resulted in the theft of millions of patient records.

Medical identity theft is often not immediately identified by patients or their provider, giving criminals years to milk such credentials. That makes medical data more valuable than credit cards, which tend to be quickly canceled by banks once fraud is detected.

Security experts say cyber criminals are increasingly targeting the $3 trillion US healthcare industry, which has many companies still reliant on aging computer systems that do not use the latest security features.

Anthem said it would send a letter and email to everyone whose information was stored in the hacked database. It also set up an informational website, www.anthemfacts.com, and will offer to provide a credit-monitoring service.