(CNN) -- "UglyGorilla," "KandyGoo," and "WinXYHappy" are some of the aliases used by the Chinese accused of hacking U.S. companies on Monday.
The men behind these handles are officers of the People's Liberation Army (PLA) under a unit known simply by the code 61398.
Little is confirmed about the mysterious unit 61398, a section that the Chinese authorities have not officially acknowledged. The Chinese defense ministry said the country's military "has never supported any hacker activities."
But the U.S. indictment notice pinpoints a non-descript building on Datong Road in Shanghai's Pudong District as one of the locations for unit 61398's alleged cyber espionage activities.
When CNN tried to visit the building last year, our correspondents were chased away by security guards, as seen in the video above.
What is unit 61398 and what do they do? U.S.-based Internet security firm Mandiant released a 60-page report last year detailing allegations against the shadowy unit over a six-year period.
According to Mandiant's document and the U.S. indictment, here's what we know about the secret division.
Mandiant says unit 61398 is also known as the "comment crew," and has systematically stolen hundreds of terabytes of data from at least 141 organizations across 20 industries worldwide since as early as 2006.
Mandiant estimates that more than 1,000 servers are being used by unit 61398.
The security firm believes the unit employs anywhere from hundreds to thousands of staff. A look at the physical size of the building in Shanghai -- 12 floors high, with more than 130,000 square feet of space -- suggests the unit could house around 2,000 people.
Mandiant observed 141 companies targeted by unit 61398, out of which 115 were in the United States. These were blue-chip companies in important industries such as aerospace, satellite and telecommunications, and information technology -- strategic industries that were identified in China's five year plan for 2011 to 2015.
"It's really a who's who of American companies," says Grady Summers, Mandiant's vice president.
Some of the alleged victims included in the latest indictment are U.S. Steel Corp., Westinghouse, Alcoa, Allegheny Technologies, the United Steel Workers Union and SolarWorld.
Unit 61398 was given a special fiber optic communication infrastructure by state-owned enterprise China Telecom in the name of national defense, Mandiant reported.
The accused Chinese hackers reportedly use spear-phishing to hack into companies. The simple trick makes scam emails appear like they are from someone the receiver actually knows. For example, the emails would be personally addressed and signed by another employee in the same company.
Spear-phishers may scan social media to find out personal details about a victim to make the scam emails seem legitimate.
Tip of the iceberg
Unit 61398 is just one of more than 20 cyber attack groups with origins in China, says Mandiant.