Chinese hackers break into Pentagon

ZHOUSHAN, China (CNN) -- They operate from a bare apartment on a Chinese island. They are intelligent 20-somethings who seem harmless. But they are hard-core hackers who claim to have gained access to the world's most sensitive sites, including the Pentagon. The leader of these Chinese hackers says there "is always a weakness" on networks that allows cyber break-ins.

In fact, they say they are sometimes paid secretly by the Chinese government -- a claim the Beijing government denies.