The U.S. has traced a massive cyberespionage operation against President Barack Obama's and Sen. John McCain's 2008 presidential campaigns back to China-backed hackers, U.S. intelligence officials have told NBC News. The revelations prompted the U.S. to issue Chinese officials high-level warnings to stop its cyberspying — and could cast a pall over a two-day summit between Obama and Chinese President Xi Jinping beginning in California on Friday. Targeted in the campaign, officials told NBC News, were internal data including policy position emails and key advisers' and aides' private emails. "Based on everything I know, this was a case of political cyberespionage by the Chinese government against the two American political parties," Dennis Blair, Obama's former director of national intelligence, told NBC News. "They were looking for positions on China." The intrusions were first discovered during summer 2008 — when Obama and McCain were on the campaign trail, and when Beijing was busy hosting the Olympics.