Computer spies have broken into the Pentagon's $300 billion Joint Strike Fighter project -- the Defense Department's costliest weapons program ever -- according to current and former government officials familiar with the attacks. Similar incidents have also breached the Air Force's air-traffic-control system in recent months, these people say. 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. The latest intrusions provide new evidence that a battle is heating up between the U.S. and potential adversaries over the data networks that tie the world together. The revelations follow a recent Wall Street Journal report that computers used to control the U.S. electrical-distribution system, as well as other infrastructure, have also been infiltrated by spies abroad.