Chinese hackers breached the computer system of the Office of Personnel Management in December, officials said Thursday, and the agency will notify some 4 million current and former federal employees that their personal data may have been compromised. The hack was the second major intrusion of the agency by China in less than a year. OPM, using new tools, discovered the breach in April, said officials at the agency who declined to comment on who was behind the hack. Other U.S. officials, who spoke on conditions of anonymity because it is an ongoing investigation, identified the hackers as being from...