In 2004, a California computer whiz named Barrett Lyon uncovered the identity of a hacker running major assaults on business websites. Without fully grasping the repercussions, he set on an investigation that led him into the heart of the Russian mob. Cybercrime was evolving. No longer the domain of small-time thieves, it had been discovered by sophisticated gangs. They began by attacking corporate websites but increasingly stole financial data from consumers and defense secrets from governments.

While Barrett investigated the cutting edge of technology crime, the U.S. government...