The hackers, believed to be based in the Chinese province of Guangdong, are thought to have stolen U.S. military secrets, including aviation specifications and flight-planning software.
The U.S. government has coined the term "Titan Rain" to describe the hackers.
"From the Redstone Arsenal, home to the Army Aviation and Missile Command, the attackers grabbed specs for the aviation mission-planning system for Army helicopters, as well as Falconview 3.2, the flight-planning software used by the Army and Air Force," Alan Paller, the director of the SANS Institute, said on Tuesday.
The team is thought to consist of 20 hackers. Paller said that the Chinese government is the most likely recipient of the information they intercepted.
"Of course, it's the government. Governments will pay anything for control of other governments' computers. All governments will pay anything. It's so much better than tapping a phone," Paller said at an event at the British Department of Trade and Industry.
Titan Rain first came to public attention this summer, when the Washington Post reported that Web sites in China were being used to target computer networks in the Defense Department and other U.S. agencies.
Time magazine later reported that Titan Rain had been counter-hacked by a U.S. security expert called Shawn Carpenter.
The ongoing attacks were particularly effective on the night of Nov. 1, 2004, said Paller, who outlined his version of how the hackers first scanned, then broke into, U.S. government computers:
At 10:23 p.m. PST, the Titan Rain hackers exploited vulnerabilities at the U.S. Army Information Systems Engineering Command at Fort Huachuca, Ariz.
At 1:19 a.m., they exploited the same hole in computers at the Defense Information Systems Agency in Arlington, Va.
At 3:25 a.m., they hit the Naval Ocean Systems Center, a Defense Department installation in San Diego, Calif.
At 4:46 a.m., they struck the U.S. Army Space and Strategic Defense installation in Huntsville, Ala.
The United Kingdom is also, according to National Infrastructure Security Co-ordination Centre. The government body cannot name the countries concerned as this may "ruin diplomatic efforts to halt the attacks," NISCC director Roger Cummings said Tuesday.
Tom Espiner of ZDNet UK reported from London.