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China's cyber spies have accessed the private emails of “many” top Obama administration officials, according to a senior U.S. intelligence official and a top secret document obtained by NBC News, and have been doing so since at least April 2010.

The email grab –- first codenamed “Dancing Panda” by U.S. officials, and then “Legion Amethyst” –- was detected in April 2010, according to a top secret NSA briefing from 2014. The intrusion into personal emails was still active at the time of the briefing and, according to the senior official, is still going on.

In 2011, Google disclosed that the private gmail accounts of some U.S. officials had been compromised, but the briefing shows that private email accounts from other providers were compromised as well.

Read An Exclusive NBC News Report on More Than 600 Chinese Hacks of U.S. Websites

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The government email accounts assigned to the officials, however, were not hacked because they are more secure, says the senior U.S. intelligence official.

The senior official says the private emails of “all top national security and trade officials” were targeted.

The Chinese also harvested the email address books of targeted officials, according to the document, reconstructing and then “exploiting the(ir) social networks” by sending malware to their friends and colleagues.

The time period overlaps with Hillary Clinton’s use of a private email account while Secretary of State from Jan. 21, 2009 to Feb. 1, 2013. The names and ranks of the officials whose emails were actually grabbed, however, were not disclosed in the NSA briefing nor by the intelligence official.

A different NSA document leaked by Edward Snowden revealed that in late 2010 China had attempted to spy on the emails of four U.S. officials, including then Joint Chiefs of Staff Chairman Adm. Mike Mullen and Chief of Naval Operations Adm. Gary Roughead. The document, posted on-line by Der Spiegel earlier this year, said the Chinese spies had tried to insert malicious software into their computers.

An NSA slide shows the organization of the Chinese government’s hacking units, with separate operations run by the Chinese military and by state security. NBC News

NBC News reported in 2013 that prior to the 2008 presidential election, Chinese cyber spies had targeted the presidential campaigns of then Sen. Obama and Sen. John McCain in order to read emails and policy papers, and had successfully compromised some emails, including private correspondence from McCain.

The “Dancing Panda” campaign revealed in the 2014 NSA briefing paper was among more than 30 “intrusion sets” launched by China that the NSA and other intelligence units identified last year in classified documents. NBC News reported last week that those campaigns resulted in more than 600 successful attacks on targets.

The NSA declined to comment.