A new investigative report says China's military is behind a massive hacking campaign against U.S. targets.

The detailed 60-page report was compiled by a U.S.-based Internet security company called Mandiant. They've traced more than 140 major cyberattacks to a unit of China's military which is based in Shanghai.

The hacking attempts targeted 20 industries, including military contractors and companies with access to energy grids.

Mandiant said the Chinese military hackers have "systematically stolen hundreds of terabytes of data."

China called the allegations "groundless." Chinese Foreign Ministry spokesman Hong Lei did not directly address the claims but said he doubted the evidence would withstand scrutiny.

But Mandiant stated in a summary of its latest report, "The details we have analyzed during hundreds of investigations convince us that the groups conducting these activities are based primarily in China and that the Chinese government is aware of them."

Mandiant said it has traced the massive amount of hacking back to one 12-story office building run by "Unit 61398" of the People's Liberation Army.