Top Democratic senator agrees her party rigged nomination for Clinton
This comes after Donna Brazile, the former Democratic National Committee (DNC) chairwoman, wrote in a new book that the party reached an "unethical" fundraising agreement with the Clinton campaign.
US President Donald Trump has called for an investigation into the allegations.
Did the Democratic party favor Clinton over Sanders?
Sanders supporters have long accused Democrats of favoring Clinton
When CNN asked Warren whether Clinton's contest against her Democratic rival Bernie Sanders was rigged, she replied: "Yes."
Warren, who had campaigned for Clinton, has called for the DNC to be more accountable.
The DNC was supposed to be neutral in the contest between Clinton and Sanders to secure its presidential nomination.
Sanders' supporters have long alleged that the DNC showed Clinton more favor.
Brazile says Clinton was bankrolling Democratic party
In her upcoming book, Brazile's has alleged that the cash-strapped DNC signed a fundraising agreement with the Clinton campaign in August 2015.
She wrote that Clinton's campaign "had the DNC on life support, giving it money every month…."
Brazile feels the agreement wasn't illegal but "compromised the party's integrity."
Clinton's defenders claim the Sanders campaign signed its own fundraising agreement with the DNC.
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Trump weighs in
Trump: "Crooked Hillary bought the DNC"
Trump seized onto Warren's revelation by tweeting: "Crooked Hillary bought the DNC & then stole the Democratic Primary from Crazy Bernie!"
He called for an investigation into the allegation saying: "Let's go FBI & Justice Dept."