In a Facebook post Saturday, Marshall said emails were written in the heat of the moment.
"I deeply regret that my insensitive, emotional emails would cause embarrassment to the DNC, the chairwoman, and all of the staffers who worked hard to make the primary a fair and open process," Marshall said. "The comments expressed do not reflect my beliefs nor do they reflect the beliefs of the DNC and its employees. I apologize to those I offended."
WikiLeaks does not say how it obtained the internal DNC files. The DNC's computer network was breached in the past by Russian government cyber operations, NBC News reported last month.
Saturday, the Clinton campaign released a statement that said: "We are very proud of the campaign that we ran. Hillary Clinton has said a number of times publicly that Sen. Sanders ran an extraordinary, hard-fought-campaign based on a real vigorous, and contested primary."
"This is further evidence the Russian government is trying to influence the outcome of the election," the Clinton campaign statement said.
Sanders endorsed Clinton earlier this month.
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Another email posted online by Wikileaks, a May 21 message purported to be from Deputy Communications Director Mark Paustenbach, asks: "Wondering if there's a good Bernie narrative for a story, which is that Bernie never ever had his act together, that his campaign was a mess. Specifically, DWS had to call Bernie directly in order to get the campaign to do things because they'd either ignored or forgotten to something critical."
The email posted on WikiLeaks also says: "It's not a DNC conspiracy, it's because they never had their act together."
The recipient of that email, Communications Director Luis Miranda, apparently replies: "True, but the Chair has been advised to not engage. So we'll have to leave it alone," according to the documents released by Wikileaks.