The U.S. Navy said that hackers accessed sensitive information for more than 130,000 current and former U.S. sailors.
The breach, announced on November 23 by Vice Admiral Robert Burke, was the latest in a string of major cybersecurity breaches that have stoked worries about computer systems and secret information being stolen.
The navy said a laptop used by a contractor was hacked, and the contractor's employer, computer giant Hewlett Packard, then informed the navy of the breach on October 27.
As many as 134,386 current and former sailors may have had their information accessed, including Social Security numbers, which could be used by hackers to commit fraud.
Major international companies and corporations have seen a string of hacking in recent years.
The U.S. presidential election campaign was dogged by intelligence agency conclusions that Russian government-backed hackers accessed the Democratic Party and campaign officials.
Based on reporting by Reuters