According to The Washington Post:
The massive hack last year of the Office of Personnel Management's system containing security clearance information affected 21.5 million people, including current and former employees, contractors and their families and friends, officials said Thursday.
That is in addition to a separate hack – also last year — of OPM's personnel database that affected 4.2 million people. That number was previously announced.
Together, the breaches arguably comprise the most consequential cyber intrusion in U.S. government history. Administration officials have privately said they were traced to the Chinese government and appear to be for purposes of traditional espionage.
Update: Office of Personnel Management Director Katherine Archuleta finally resigned mid-Friday.
(Score: 2, Informative) by kc on Friday July 10 2015, @11:57PM
OPM does investigations on all (or nearly all?) Federal employees and contractors, even those that don't need a "security clearance". It looks like this data includes all of those records. For example, see https://www.opm.gov/cybersecurity/ [opm.gov] . It does not say that only records of people with security clearance were compromised.