Wired has a good, long article on how millions of security clearance documents were stolen from the Office of Personnel Management:
Once Captain America’s name popped up, there could be little doubt that the Office of Personnel Management had been hit by an advanced persistent threat (APT)—security-speak for a well-financed, often state-sponsored team of hackers. APTs like China’s Unit 61398 have no interest in run-of-the-mill criminal activities such as selling pilfered Social Security numbers on the black market; they exist solely to accumulate sensitive data that will advance their bosses’ political, economic, and military objectives.
The hackers...delved into the complete personnel files of 4.2 million employees, past and present. Then, just weeks before OPM booted them out, they grabbed approximately 5.6 million digital images of government employee fingerprints.