The fix was in after all.

Last year's presidential primary between Hillary Clinton and Bernie Sanders was rigged by the Democratic National Committee - just as Sanders' supporters suspected - to hand the nomination to Clinton, according to a bombshell claim by Donna Brazile. The onetime Clinton confidante, CNN commentator and former interim party boss made the explosive claim Thursday while touting a new book that could sever her ties to Team Clinton for good.

“I had promised Bernie when I took the helm of the Democratic National Committee after the convention that I would get to the bottom of whether Hillary Clinton’s team had rigged the nomination process, as a cache of emails stolen by Russian hackers and posted online had suggested,” Brazile wrote in a piece for Politico Magazine.

“By Sept. 7, the day I called Bernie, I had found my proof and it broke my heart,” Brazile said.

The proof, according to Brazile, was a joint fundraising agreement document between the DNC, the Hillary Victory Fund and Hillary for America. It had been signed in August 2015, four months after Clinton announced her candidacy and a year before she officially secured the nomination over Sanders.

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“The agreement—signed by Amy Dacey, the former CEO of the DNC, and Robby Mook with a copy to Marc Elias—specified that in exchange for raising money and investing in the DNC, Hillary would control the party’s finances, strategy, and all the money raised,” Brazile wrote. “Her campaign had the right of refusal of who would be the party communications director, and it would make final decisions on all the other staff.”

Brazile took over as the interim DNC chairman in 2016 when Florida Rep. Debbie Wasserman Schultz was forced out as chairman over the emails, which indicated the party organization unfairly favored Clinton over Sanders during the primary.

The ardent supporters of Sanders, the Democratic socialist, have long accused the party establishment of actively taking steps to benefit Clinton during the primary.

In a statement to Fox News, Wasserman Schultz didn’t directly address Brazile’s claims, but defended her tenure leading the committee.

"It was a tremendous honor to be asked by President Obama to serve as chair of the DNC,” Wasserman Schultz said Thursday. “I am proud of the work our team did to support Democrats up and down the ballot in the 2016 election and to re-elect the president in 2012.”

DNC spokesman Xochitl Hinojosa, who works under current DNC chairman Tom Perez, told Fox News the party’s official policy is to not take sides during the primaries.

“The DNC must remain neutral in the presidential primary process, and there shouldn’t even be a perception that the DNC is interfering in that process,” Hinojosa said.

She said joint fundraising committees were created between the DNC and the Clinton and Sanders campaigns during the 2016 cycle but Clinton was the only candidate who raised money for the party.

The piece, titled “Inside Hillary Clinton’s secret takeover of the DNC,” is excerpted from Brazile’s forthcoming book, which is being published this month.

After taking over, she said she went in search for “evidence of internal corruption that would show that the DNC was rigging the system to throw the primary to Hillary.”

“I had been wondering why it was that I couldn’t write a press release without passing it by Brooklyn,” she wrote. “Well, here was the answer.”

It’s typical, she said, for candidates to make such arrangements with party organizations – but after they’ve won the nomination. Brazile said the arrangement does not appear to be “illegal, but it sure looked unethical.”

“If the fight had been fair, one campaign would not have control of the party before the voters had decided which one they wanted to lead,” she said. “This was not a criminal act, but as I saw it, it compromised the party’s integrity.”

Sanders has not yet commented on Brazile’s story, but other progressives are expressing outrage.

During an afternoon appearance on CNN, Democratic Massachusetts Sen. Elizabeth Warren, another favorite of the liberal base, said “yes” when asked if she thinks the primary was rigged.

“This is a real problem,” she told anchor Jake Tapper. “What we’ve got to do, as Democrats, now is we’ve got to hold this party accountable.”

Fox News' Brooke Singman and Dave Sweet contributed to this report.