The sophisticated Russian groups, which have previously targeted the White House, the State Department and the Joint Chiefs of Staff, specifically concentrated on the DNC's research units and had access to all of the committee's internal communications, including chat and email applications.
The DNC's opposition research unit, which sources indicate was specifically targeted by the hackers, is tasked with compiling unflattering information on Republican opponents -- particularly presumptive Republican nominee Donald Trump -- to potentially use against them in the course of a political campaign
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Opposition research could include everything from old press clippings to footage of candidates on the stump recorded by party operatives to obscure legal and tax documents collected through public records requests. Much of this sensitive information may never come out during a campaign, and could be of use to the Russian government in seeking to embarrass or apply pressure to targets.
Trump spokeswoman Hope Hicks directed questions to the United States Secret Service.
 
 The DNC called in computer security company CrowdStrike during the first week of May to investigate the suspected hack. CrowdStrike found that two Russian intelligence agencies - which may not have been working together - breached the network in summer 2015 and April 2016, respectively.
"The security of our system is critical to our operation and to the confidence of the campaigns and state parties we work with," said Rep. Debbie Wasserman Schultz (D-Fla.), the chairwoman of the DNC. "When we discovered the intrusion, we treated this like the serious incident it is and reached out to CrowdStrike immediately. Our team moved as quickly as possible to kick out the intruders and secure our network."
Shawn Henry, the president of CrowdStrike, said that foreign espionage operations commonly seek information about political campaigns.
"We know with certainty that foreign intelligence services are constantly interested in political processes. They're interested in strategies, they're interested in foreign policy," he said. "The DNC and other NGOs have been targeted over the years by this very, very sophisticated group with a high degree of capability and some very sophisticated technology."
Director of National Intelligence James Clapper warned last month that foreign hackers were likely aiming to target U.S. presidential candidates.
                      
"We have already had some indications of that, and a combination of DHS, FBI are doing what they can to educate both candidates of potential cyber threats," he said, adding that "I anticipate as the campaigns intensify we will probably have more of it."
Hackers have targeted political campaigns in previous presidential elections. Both Barack Obama and John McCain's campaigns were hacked in 2008. Chinese hackers targeted both Obama's reelection effort and Mitt Romney's 2012 campaign.
Opposition research could include everything from old press clippings to footage of candidates on the stump recorded by party operatives to obscure legal and tax documents collected through public records Requests.
Much of this sensitive information may never come out during a campaign, and could be of use to the Russian government in seeking to embarrass or apply pressure to targets