Hillary ClintonHillary Diane Rodham ClintonHarris readies a Phase 2 as she seeks to rejuvenate campaign Nevada Senate passes bill that would give Electoral College votes to winner of national popular vote 2020 Dems break political taboos by endorsing litmus tests MORE's campaign manager on Sunday questioned whether Russia is behind the leaked emails that show Democratic Party officials privately planned how to undermine the presidential campaign of Sen. Bernie SandersBernie SandersHarris readies a Phase 2 as she seeks to rejuvenate campaign 2020 Dems put spotlight on disabilities issues Lee, Sanders introduce bill to tax Wall Street transactions MORE (I-Vt.). 

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"What's disturbing about this entire situation is that experts are telling us that Russian state actors broke into the DNC, took all those emails and are now leaking them out through these websites," Robby Mook said on ABC's "This Week," referring to the Democratic National Committee. 

"It's troubling that some experts are telling us this was done by the Russians for the purpose of helping Donald TrumpDonald John TrumpFeinstein, Iranian foreign minister had dinner amid tensions: report The Hill's Morning Report - Trump says no legislation until Dems end probes Harris readies a Phase 2 as she seeks to rejuvenate campaign MORE." 

Mook also criticized an email that appears to show DNC Chief Financial Officer Brad Marshall questioning Sanders's religion. 

"Nobody in politics should be attacked for their religion, ever,"  Mook said, noting that Marshall apologized for the email.  

In the emails leaked Friday by WikiLeaks, Marshall wrote: “It might may no difference, but for KY and WVA can we get someone to 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."

The leak comes ahead of this week's Democratic National Convention in Philadelphia, where Clinton will accept the party's nomination for president.  

"I think the DNC needs to get to the bottom of the facts and take appropriate action on these emails," Mook said Sunday.