In the case of the fighter-jet program, the intruders were able to copy and siphon off several terabytes of data related to design and electronics systems, officials say, potentially making it easier to defend against the craft…

Attacks like these — or U.S. awareness of them — appear to have escalated in the past six months, said one former official briefed on the matter. “There’s never been anything like it,” this person said, adding that other military and civilian agencies as well as private companies are affected. “It’s everything that keeps this country going.”…

Six current and former officials familiar with the matter confirmed that the fighter program had been repeatedly broken into. The Air Force has launched an investigation…

Investigators traced the penetrations back with a “high level of certainty” to known Chinese Internet protocol, or IP, addresses and digital fingerprints that had been used for attacks in the past, said a person briefed on the matter.

As for the intrusion into the Air Force’s air-traffic control systems, three current and former officials familiar with the incident said it occurred in recent months. It alarmed U.S. national security officials, particularly at the National Security Agency, because the access the spies gained could have allowed them to interfere with the system, said one former official. The danger is that intruders might find weaknesses that could be exploited to confuse or damage U.S. military craft.