In early February, shortly before the New Hampshire primary, Sanders’s attendance was the subject of several news stories suggesting that mingling with wealthy donors was at odds with calls to rein in the political influence of Wall Street and other corporate interests. Sanders, officially an independent, said he was merely trying to help get more Democrats elected to the Senate.

Among the email contained in the latest batch released by WikiLeaks, the anti-secrecy organization, is one written by Tina Flournoy, Bill Clinton’s chief of staff, to Brian Fallon, Hillary Clinton’s national press secretary. It appears to have included a picture and the description: “Bernie at the DSCC retreat.”

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“Omg,” Fallon responded.

John Podesta, Clinton’s campaign chairman, then weighed in, asking: “Can we tweet?”

Fallon responded: “I think we should give it to the NY post.”

Flournoy identified the site of the retreat and pledged to track down the exact date.

In the following days, both Bill and Hillary Clinton picked up on the attack on Sanders, suggesting hypocrisy on his part.

After Hillary Clinton secured the Democratic nomination, she was endorsed by Sanders, who is now campaigning on her behalf.

Clinton’s campaign has declined to verify the authenticity of the hacked email from Podesta’s account, instead emphasizing that Russian hackers appear to have provided the documents to WikiLeaks.

"We now know the FBI believes the Russians are behind this hack," Clinton spokesman Glen Caplin said Thursday, adding that Roger Stone, an associate of Republican nominee Donald Trump, has acknowledged "back-channeling" with Julian Assange, the founder of WikiLeaks.