Hackers from China have been making the news recently as a number of American companies and government agencies have been the victims snooping. But as Michael Isikoff of NBC News reports, attacks from behind the Great Firewall have been happening for a while now:

The U.S. secretly traced a massive cyberespionage operation against the 2008 presidential campaigns of Barack Obama and John McCain to hacking units backed by the People’s Republic of China, prompting high level warnings to Chinese officials to stop such activities, U.S. intelligence officials tell NBC News.

The disclosure on the eve of a two-day summit between the U.S. and Chinese presidents highlights what has become a persistent source of tension between the two global powers: Beijing’s aggressive, orchestrated campaign to pierce America’s national security armor at any weak point – in this case the computers and laptops of top campaign aides and advisers who received high-level briefings.

As for China’s response to being caught, Isikoff reports:

The 2008 attacks, for example, prompted U.S. intelligence officials to sternly warn the Chinese that they had “crossed the line,” says one former senior U.S. official who was directly involved in the investigation.

“We told them we knew what they were up to – and that this had gone too far,” said the former official. Chinese officials listened politely and denied they had anything to do with the attacks on the campaign, the former official said.