 -- WikiLeaks leaked nearly 20,000 emails on Friday from top Democratic National Committee officials, exchanged from January 2015 through May 2016. Several emails released show that although the DNC was supposed to remain neutral during the primary contest, officials grew increasingly agitated with Bernie Sanders and his campaign, at some points even floating ideas about ways to undermine his candidacy.

The source of the leak has not been revealed, though Hillary Clinton's campaign manager, Robby Mook, said on ABC News' "This Week With George Stephanopoulos" on Sunday that he believes the Russians were instrumental in it.

"Experts are telling us that Russian state actors broke into the DNC, took all these emails and now are leaking them out through these websites," Mook said Sunday. "It's troubling that some experts are now telling us that this was done by the Russians for the purpose of helping Donald Trump."

Regardless of who was behind the leak, the fallout for the DNC has been severe. Just one day before the Democratic convention was set to begin, DNC Chairwoman Debbie Wasserman Schultz announced her resignation, effective at the end of the week. And as expected, Sanders supporters, hundreds of whom are delegates at the convention, are furious about the content of the emails.

Here are some of the most damaging finds from the leak:

DNC Chairwoman Debbie Wasserman Schultz Calls Sanders Campaign Manager Jeff Weaver an "A--" and a "Liar"

In May the Nevada Democratic State Convention became rowdy and got out of hand in a fight over delegate allocation. When Weaver went on CNN and denied any claims violence had happened, Debbie Wasserman Schultz, once she was notified of the exchange, wrote "Damn liar. Particularly scummy that he never acknowledges the violent and threatening behavior that occurred."

In another instance, right before the Nevada Convention, Weaver publicly commented, "I think we should go to the [national] convention." The chairwoman was flagged about this comment and responded in an e-mail, "He is an a--."

Highlighting Sanders' Faith

One email shows that a DNC official contemplated highlighting Sanders' alleged atheism — even though he has said he is not an atheist — during the primaries as a possibility to undermine support among voters.

"It may make no difference but for KY and WA can we get someone to ask his belief," Brad Marshall, CFO of the DNC, wrote in an email on May 5, 2016. "He had skated on having a Jewish heritage. 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."

Building a Narrative Against Sanders

"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," DNC National Secretary Mark Paustenbach wrote in an email to National Communications Director Luis Miranda on May 21. After detailing ways in which the Sanders camp was disorganized, Paustenbach concludes, "It's not a DNC conspiracy it's because they never had their act together."

The idea was nixed, though. "True," Miranda acknowledged in his response. "But the chair has been advised not to engage. So we'll have to leave it alone."

Lamentations That Sanders Is Not a Democrat

As the primary season wore on, Wasserman Schultz appeared to grow exasperated with Sanders' desire to stay in the race when the delegate math was against him — in one email lamenting the fact that he is an independent in the Senate but was running as a Democrat in the primaries. In an April 24 email she received with an article describing the ways Sanders felt the DNC was undermining his campaign, she wrote back, "Spoken like someone who has never been a member of the Democratic Party and has no understanding of what we do."