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The chief financial officer of the Democratic National Committee on Saturday apologized for the "insensitive" contents of an email leaked by the website WikiLeaks which appears to refer to Bernie Sanders.

The internal email from Brad Marshall was one of more than 19,000 emails from several officials in the Democratic National Committee released Friday by the website founded by Julian Assange.

An email purportedly written by Marshall in May appears to refer to Sanders, but does not name him.

The email says "ask his belief. Does he believe in a God. He had skated on saying he has a Jewish heritage. I think I read he is an atheist. This could make several points difference with my peeps. My Southern Baptist peeps would draw a big difference between a Jew and an atheist."

In a Facebook post Saturday, Marshall said emails were written in the heat of the moment.

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"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."

U.S. Sen. Bernie Sanders endorses Democratic U.S. presidential candidate Hillary Clinton during a campaign rally in Portsmouth, New Hampshire, U.S., July 12, 2016. BRIAN SNYDER / Reuters

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, an official with the campaign of presumptive Democratic nominee Hillary Clinton 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 official said.

Sanders endorsed Clinton earlier this month.

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.

May 21 is the day that Sanders announced his support for Tim Canova, who is running against Democratic National Committee Chair Debbie Wasserman Schultz in an Aug. 30 primary for her House seat.

NBC News reached out to the DNC Saturday but did not receive a response.

Some Sanders supporters have claimed that the DNC was biased against the Sanders campaign, and that Wasserman Schultz tried to influence the outcome. Wasserman Schultz has denied the claims.