Story highlights Underground network of North Korean hackers are operating in China, defector says

They're based in Shenyang, defector says, near the North Korean border

North Korean embassy in Beijing and other government officials declined to comment

Shenyang, China (CNN) On the streets of the neon-lit Chinese city of Shenyang, you'll find a restaurant, hotel, and other businesses owned and operated by the North Korean government.

You'll also find a secret network of North Korean hackers, known as Bureau 121, according to defector Kim Heung-Kwang.

"It's easy for them to work secretly. It also has great Internet infrastructure," says Kim Heung-kwang, a former Pyongyang computer science professor who escaped North Korea in 2004.

Kim says some of his own students became cyber warriors for the hacker network.

"By day, they worked regular jobs. But the rest of the time, they were acting on orders from Pyongyang," he says.

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