"It would reflect the idea that this is becoming a more routine activity and that they are quite methodical," Clemente said.
The PLA's Third Department is brimming with resources, according to studies commissioned by the U.S. government, with 12 operation bureaus, three research institutes, and an estimated 13,000 linguists, technicians and researchers on staff. It's further reinforced by technical teams from China's seven military regions spread across the country, and by the military's vast academic resources, especially the PLA University of Information Engineering and the Academy of Military Sciences.
The PLA is believed to have made cyber warfare a key priority in its war-fighting capabilities more than a decade ago. Among the few public announcements of its development came in a May 25, 2011 news conference by Defense Ministry spokesman Geng Yansheng, in which he spoke of developing China's "online" army.
"Currently, China's network protection is comparatively weak," Geng told reporters, adding that enhancing information technology and "strengthening network security protection are important components of military training for an army."
Unit 61398 is considered just one of many such units under the Third Department responsible for hacking, according to experts.
Greg Walton, a cyber-security researcher who has tracked Chinese hacking campaigns, said he's observed the "Comment Crew" at work, but cites as equally active another Third Department unit operating out of the southwestern city of Chengdu. It is tasked with stealing secrets from Indian government security agencies and think tanks, together with the India-based Tibetan Government in Exile, Walton said.
Another hacking outfit believed by some to have PLA links, the "Elderwood Group," has targeted defense contractors, human rights groups, non-governmental organizations, and service providers, according to computer security company Symantec.
It's believed to have compromised Amnesty International's Hong Kong website in May 2012, although other attacks have gone after targets as diverse as the Council on Foreign Relations and Capstone Turbine Corporation, which makes gas microturbines for power plants.
Civilian departments believed to be involved in hacking include those under the Ministry of Public Security, which commands the police, and the Ministry of State Security, one of the leading clandestine intelligence agencies. The MSS is especially suspected in attacks on foreign academics studying Chinese social issues and unrest in the western regions of Tibet and Xinjiang.
Below them on the hacking hierarchy are private actors, including civilian universities and research institutes, state industries in key sectors such as information technology and resources, and college students and other individuals acting alone or in groups, according to analysts, University of Maryland's Forno said.
China's government isn't alone in being accused of cyber espionage, but observers say it has outpaced its rivals in using military assets to steal commercial secrets.
"Stealing secrets is stealing secrets regardless of the medium," Forno said. "The key difference is that you can't easily arrest such electronic thieves since they're most likely not even in the country, which differs from how the game was played during the Cold War."