Chinese cyber spies have been reading the private emails of "many" top Obama administration officials since at least 2010, according to a top U.S. intelligence official and top secret document obtained by NBC News.

The private email accounts of "all top national security and trade officials" were targeted. Officials' government email accounts, however, were more secure and therefore not hacked, according to the senior intelligence source.

"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," explains NBC. "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."

The hackers were also able to acquire the email address books of officials, which allowed them to reconstruct and exploit their social networks by sending malware to colleagues and friends, according to NBC.

The hacking coincided with then-Secretary of State Hillary Clinton's exclusive use of a private unsecured email account and home-based server to conduct all official federal business from 2009 to 2013. NBC didn't confirm whether Clinton is among the hacked officials, but a number of security experts and even a former CIA deputy director have said they are near certain Clinton's account was hacked by multiple foreign governments.

As Gizmodo notes, "Clinton relied far more heavily on her personal email address while she was in office than other officials, which means the contents of her emails likely contain more officially compromising information."

Just last week, Judge Andrew Napolitano told Fox News that Clinton sent several emails that revealed the location of U.S. Ambassador Christopher Stevens in Libya, as well as the location of fighter jets during NATO bombing runs, as HNGN reported.

Two U.S. inspectors general recently examined a random limited selection of 40 of Clinton's emails that were turned over to the State Department and discovered that four of them contained classified information. The emails "were classified when they were sent and are classified now," spokeswoman Andrea Williams told CNN. State Department officials told the inspectors general that "there are potentially hundreds of classified emails within the approximately 30,000 provided by former Secretary Clinton."

The FBI is reportedly conducting a criminal probe into Clinton's private system to determine the extent to which she sent and stored classified documents.