The Pentagon suffered the most significant breach yet of its cyber security system, underscoring fears that secret defense files and networks are vulnerable to cyber attacks.

Deputy Secretary of Defense William Lynn said that hackers stole 24,000 sensitive defense department files in a single March operation. He said that the Pentagon believes the attack came from a nation, not an individual, but would not provide further details, saying only that officials had a pretty good idea of who was responsible. He added that previous cyber attacks are believed to have emanated from Russia or China, but that hostile terrorist groups represent a greater threat.

If a terrorist group gains disruptive or destructive cybertools, we have to assume they will strike with little hesitation, Lynn said in a speech at the National Defense University.

A spate of cyber attacks on various government entities, from the U.S. Senate to the Central Intelligence Agency, has spurred a wave of arrests, including a 19-year-old Ryan arrested in Essex for his alleged role in the LulzSec group's believed to have attacked the CIA website. They have also sharpened awareness of the vulnerability of online data and led organizations to take more security measures. At his Senate confirmation hearing last month, new Defense Secretary Leon Panetta cited a strong likelihood that the next Pearl Harbor would be a debilitating cyber attack aimed at government networks or energy grids, and he has vowed to make bolstering cyber security a priority

Lynn added to those fears, noting that cyber attacks have already targeted secrets relating to surveillance technologies and military technology like unarmed drones. According to military's cyber command, about 250,000 potential attackers scope out the Pentagon's defenses each hour.

Marine Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, said that our networks are realy our lifeblood and noted that the Pentagon's cyber warfare on strategy is focused primarily on protecting itself. If it's OK to attack me and I'm not going to do anything other than improve my defenses every time you attack me, it's difficult to deter future attacks, Cartwright said.

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