Many hackers working directly for the Chinese government are men in their 20s and 30s who have been trained at universities run by the People’s Liberation Army and are employed by the state in myriad ways. Those working directly for the military usually follow a 9-to-5 weekday schedule and are not well paid, experts and former hackers said. Some military and government employees moonlight as mercenaries and do more hacking on their own time, selling their skills to state-owned and private companies. Some belong to the same online social networking groups.

“There are many types of relationships,” said Adam Segal, a China and cybersecurity scholar at the Council on Foreign Relations in New York. “Some P.L.A. hackers offer their services under contract to state-owned enterprises. For some critical technologies, it is possible that P.L.A. hackers are tasked with attacks on specific foreign companies.”

The Obama administration makes a distinction between hacking to protect national security, which it calls fair play, and hacking to obtain trade secrets that would give an edge to corporations, which it says is illegal. China and other nations accuse the United States of being the biggest perpetrator of both kinds of espionage.