More than 12.6 million U.S. consumers fell victim to identity fraud last year, incurring costs of more than $21 billion -- a three-year high, according to a study published Wednesday.

According to Javelin Strategy and Research's 2013 Identity Fraud Report, there were more incidents of identity fraud in 2012 than in any other year of the past decade except one. An incident of identity fraud occurs once every three seconds in the U.S. alone, the study said.

New account fraud -- in which a criminal uses stolen identity information to open an account at a store or bank -- represents the largest growth segment in the fraud study, according to Jim Van Dyke, CEO at Javelin.

"The problem with new account fraud is that it's a huge blind spot for most consumers," Van Dyke said. "When an account is opened in your name, you're not going to see it on your credit card or in your bank account. Most people don't know it's happening."

But there are warning signs that consumers should pay attention to, Javelin says.