Chinese hackers apparently breached a federal database at the US Office of Personnel Management. The database contains personal information of federal employees. The hackers seemed to be targeting thousands of employees who have top secret clearances.
According to an article in the New York Times, The hackers did get access to the database in March before the attacker was detected and blocked, but a senior official from the Department of Homeland Security announced that no data was successfully extracted. According to another official the source of the attack seemed to be China.
Attacks like this provide hackers with a list of specific contacts to target in future attacks. Being a federal list of clearance holders, the list might have contained both federal and military personnel. If the hackers were able to obtain the contact information of specific Top Secret clearance holders, they would then most likely target these individuals with social engineering attacks to gain more information from the users or even try to gain access to their companies, departments or units.