The revelation raises questions about whether the DNC could have done more to limit the damage done by hackers.
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Democratic Party officials say Russian intelligence agents are suspected in the hacking.
Officials say the hackers were inside the DNC network for about a year, and were kicked out of the servers in June.
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Back in April, the DNC brought in consultants from Crowdstrike, a private security firm.
Over the weekend, Wikileaks began publishing emails which appear to show how DNC officials worked to undermine the presidential campaign of Vermont Sen. Bernie Sanders in favor of a nomination for Secretary Hillary Clinton.
The Clinton campaign pointed fingers at the Russians, saying the nation intended the intrusion in support of Donald Trump's campaign.
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Trump called the claim "a joke."
The FBI said Monday it was opening an investigation into the hack at the DNC.
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