Many computer security experts have called into question the United States's claim that North Korea was behind the Sony hack in November.

But new details may explain why U.S. officials have been so steadfast in blaming the Asian country.

"This attack was notable because it actually destroyed hard drives and servers at Sony," said David Sanger, the New York Times national security correspondent, who has been writing about the hack. He is also the author of Confront and Conceal: Obama's Secret Wars and Surprising Use of American Power.

The U.S. has been exploring North Korea's cyber program since 2010, Sanger says, a program that itself dates back to the early 1990s.

"At some point North Korean generals came to the conclusion, after sending some of their younger hackers off to China, that this could also be used as a weapon, and so they began to do what the United States, Russia, China and Iran have done, which is develop a pretty good-sized cyber arsenal," said Sanger.