NBC News
The hackers were expelled from the systems over the weekend, according to The Washington Post, which broke the story. Before they were removed, the hackers downloaded the entire database of opposition research on Republican presidential candidate Donald Trump and the organization's internal communications.
The revelation comes just a month after the nation's top spy warned that hackers were indeed spying on both candidates.
"We've already had some indications of that," James Clapper, the director of national intelligence, said in May. "As the campaigns intensify, we'll probably have more."
A spokesperson for the Hillary Clinton presidential campaign did not immediately respond to a request for comment.
The Post reported that the FBI and cybersecurity firm Crowdstrike had been investigating the breach. In detailing its investigation on its blog, Crowdstrike CTO Dmitri Alperovitch said that two separate Russian government-linked hacking groups had access to the DNC network.
"Our team considers them some of the best adversaries out of all the numerous nation-state, criminal and hacktivist/terrorist groups we encounter on a daily basis," Alperovitch wrote. "Their tradecraft is superb, operational security second to none and the extensive usage of 'living-off-the-land' techniques enables them to easily bypass many security solutions they encounter."
The intrusion is not totally surprising: The Chinese government reportedly hacked both Obama and McCain in 2008, and hackers tried repeatedly to break into the campaign accounts of Obama and Romney in 2012.
Hackers working for foreign governments can gain valuable insight into a presidential candidate's mindset before they take office, or uncover private communications that might give their country a leg up in diplomatic negotiations. In 2008, for example, a letter Sen. John McCain sent to the president of Taiwan was intercepted by hackers from China.