The US has announced criminal charges against five Chinese army hackers for stealing trade secrets from American companies, the first time the US has formally accused another nation of economic espionage. Attorney General Eric Holder released the indictment, which lists 31 charges of economic espionage, trade secret theft, identity theft, conspiracy to commit computer fraud, and other related crimes, against five named officers of China's People's Liberation Army.

The indictment also lists six companies that allegedly had secrets stolen: Alcoa (aluminum), Westinghouse (nuclear energy), SolarWorld (solar energy), US Steel, Allegheny Technologies Incorporated (metals), and the US Steelworkers Union. Secrets stolen included pricing, cost, strategy, and plant designs.

The case has been in the works for a long time. The Justice Department began training prosecutors in 2012 to prepare them for confronting cyber espionage and related crimes.

Cyber espionage is seen as a major threat to economic and national security. In addition to bolstering foreign companies with valuable US technology and business secrets, the knowledge could allow enemies to build better weapons and plan attacks on critical US infrastructure.

"We do not collect intelligence to provide competitive advantage to the US or to the US economic sector," Holder said in a press conference. The economy is "directly linked to national security," he added. "The success of American companies since our nation’s founding has been the result of hard work... this is how it ought to be across the globe."

It's the first time the US has charged a country with economic espionage

China has been the most aggressive in using hackers to steal US trade secrets, according to a national intelligence report. Other offenders include Russia and Iran. This cyber espionage costs the US economy somewhere between $24 billion and $120 billion annually, according to experts' estimates.

Chinese hacker groups, some of which have been tied to the government, have reportedly targeted defense contractorsUS presidential campaignsAmerican universitiesnewspapersGoogle, and many other US-based organizations.

The US isn't coming at this issue with an entirely clear conscience, however. The government-made Stuxnet worm, reportedly targeted Iranian nuclear facilities. Holder says it isn't hypocrisy because the US does not engage in spying for commercial gain.

China denied the charges, saying they were "made up," according to the BBC, and would "damage Sino-American cooperation and mutual trust." In response, China also canceled a US-China cybersecurity forum.

It's unclear whether the accused will ever actually stand trial in the US, but officials stressed that more indictments of this type will be forthcoming. "This is the new normal," said Robert Anderson, executive associate director of the FBI.

Update, 12:33PM: This post has been updated to include new information from the Justice Department and on China's response.