Cyber attackers working for the Chinese government accessed the private emails of “many” top officials in the Obama administration.

The revelation comes from a senior U.S. intelligence official and a top secret document analyzed by NBC News. China has been reading the private emails since April 2010, writes Kate Knibbs for Gizmodo.

China Reading Obama Administration Emails After Cyber Attacks

China reading officials’ private emails since 2010

U.S. officials first codenamed the email intrusion “Dancing Panda,” before it was renamed “Legion Amethyst.” An NSA briefing from 2014 reveals that the breach was first discovered in April 2010, was ongoing in 2014, and NBC’s source claims it continues to this day.

Google revealed that Gmail accounts maintained by some U.S. officials had been compromised in 2011, however accounts from other service providers were also hacked. None of the official government email accounts were accessed due to their higher levels of security, said the NBC source.

Chinese cyber attacks targeted the private emails of “all top national security and trade officials,” the source continued.

Email address books were taken by the hackers, before they began “exploiting the(ir) social networks” by sending malicious files to the contacts. The revelation comes at a time when the FBI is investigating Hillary Clinton’s use of a private email account during her time as Secretary of State.

Clinton presidential campaign could be affected

Clinton set up a private email server at her home which was outside of government control. At the same time she used a private email account to handle some official business during her tenure as Secretary of State from 21 January 2009 to 1 February 2013.

The NSA document does not disclose the exact names of the officials that had their emails compromised, and neither did the security official. However if she was among those affected by the hack, it could significantly affect her presidential bid.

It is known that Clinton used her personal email address more frequently than other officials during her time in office, and therefore her emails may contain more sensitive information.

Revelations cause greater concerns over cyber security

In 2010, former NSA contractor Edward Snowden leaked a document which detailed China’s attempts to infiltrate the emails of 4 U.S. officials. Among their number were then Joint Chiefs of Staff Chairman Adm. Mike Mullen and Chief of Naval Operations Adm. Gary Roughead.

German daily Der Speigel posted the document online earlier this year, revealing that Chinese spies tried to infect the officials’ computers with malware.

A 2013 NBC News report detailed how Chinese cyber attackers targeted the presidential campaigns of candidates Barack Obama and John McCain. They successfully compromised selected emails, such as McCain’s private correspondence.

The so-called “Dancing Panda” campaign was one of over 30 Chinese “intrusion sets” identified by the NSA and other intelligence agencies last year, which led to over 600 successful cyber attacks.

Cyber espionage is a hugely important subject which has soured relations between Washington and Beijing. Although no public pronouncement has been made, it is thought that China was responsible for the recent hack of the Office of Personnel Management.

Hackers stole sensitive data related to millions of federal employees, and it is thought that China may be amassing a huge database which could be used to blackmail staff with security clearances and recruit a network of informers.

The issue of cyber security is not one which is going to disappear any time soon, and is sure to be on the agenda during the impending visit of Chinese President Xi Jinping to the United States.