The FBI on Wednesday issued a private warning to industry that a group of highly skilled Chinese government hackers was in the midst of a long-running campaign to steal valuable data from U.S. companies and government agencies. "These state-sponsored hackers are exceedingly stealthy and agile by comparison with the People's Liberation Army Unit 61398 ... whose activity was publicly disclosed and attributed by security researchers in February 2013," said the FBI in its alert, which referred to a Chinese military hacker unit exposed in a widely publicized report by the security firm Mandiant.