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The Department of Justice said it will charge five members of the Chinese military for hacking into American companies, marking the first time that U.S. is taking legal action against employees of a foreign government over cyber crimes.

One unnamed military source told NBC that those accused "used military and intelligence facilities to commit cyber espionage against U.S. companies." U.S. officials have long pointed to China as the source of cyber attacks on American firms, per NBC, but never made such concrete charges before.

"Chinese actors are the world's most active and persistent perpetrators of economic espionage," said the Office of the National Counterintelligence Executive, a U.S. government agency, in a 2011 report. A year ago, several U.S. newspapers, including The New York Times and The Wall Street Journal, said hackers traced to China attacked their newsroom computer systems.

The accusations have flown both ways. The Washington Post reported back in March that China was not pleased with the U.S. government's treatment of its computer systems:

Tensions over U.S. cyber operations intensified again last weekend after a report that the NSA had penetrated the networks of a Chinese telecommunications giant, Huawei Technologies, in search of evidence that it was involved in espionage operations for Beijing and to use its equipment to spy on adversaries such as Iran. After the disclosure, first reported by the New York Times and Der Spiegel, China demanded a halt to any such activity and called for an explanation.

Attorney General Eric Holder is set to announce more details of the criminal charges later today. The Wall Street Journal reports that the individuals charged apparently work for Unit 61398 of the People's Liberation Army in Shanghai, and stole proprietary information from yet-unnamed firms — including nuclear power plant designs, and information about solar panel cost and pricing.

Update 10:35 a.m.: In a press conference on Monday, Attorney General Eric Holder named the six companies affected by the hack: Westinghouse, Alcoa, U.S. Steel, the United Steelworkers Union (USW), Allegheny Technologies Inc. and SolarWorld. Holder called the indictment a "groundbreaking step forward in addressing" the threat of cyber security. "Enough is enough," he added.