Six Democratic presidential candidates called on Comcast this week to conduct an independent investigation into how NBC handled allegations of sexual assault and harassment at the network.

Ahead of Wednesday’s presidential debate sponsored by MSNBC, Sens. Cory Booker, Kamala Harris, Bernie Sanders, and Elizabeth Warren urged the Democratic National Committee to ensure that Comcast and NBC News “take steps to clean up the toxic culture” at their networks. Rep. Julian Castro and businessman Tom Steyer later signed on to the letter to the DNC, according to UltraViolet, a national women’s group that organized the letter.

“We, the undersigned candidates, are very concerned about the message it would send to sexual assault survivors if our next debate is sponsored by MSNBC without clear commitments from Comcast, the parent company of NBC and MSNBC, to conduct an independent investigation into the toxic culture that enabled abusers and silenced survivors,” the candidates said in the letter dated Monday.

The candidates did not threaten to skip the fifth Democratic primary debate, which took place Wednesday night in Atlanta. Castro did not qualify for the debate.

NBCUniversal has come under fire for its handling of sexual-assault allegations against former Today cohost Matt Lauer, who was fired in 2017. Lauer was accused of raping an NBC colleague, Brooke Nevils, in New Yorker staff writer Ronan Farrow’s new book, Catch and Kill, released in October. Lauer has denied any wrongdoing, saying it was a consensual sexual encounter.

Farrow also alleges that NBC managers were aware that Lauer behaved inappropriately before firing him in 2017 and that the network obstructed his reporting on allegations of sexual assault by former film producer Harvey Weinstein.

NBC has said it fired Lauer as soon as managers learned of a sexual-misconduct complaint against Lauer, and has said Farrow’s reporting on Weinstein was “not ready for air.” Farrow’s reporting in the New Yorker uncovered sexual-abuse allegations against Weinstein and won the 2018 Pulitzer Prize for Public Service.

The network also conducted an internal investigation that found NBC News executives weren’t at fault because there were no previous formal complaints. Critics have said NBC should hire an outside firm to conduct an investigation.

NBC declined to comment on the presidential candidates’ letter Wednesday but referred to previous comments it made to the Hollywood Reporter.

"There is no additional investigation being launched. We are very confident in the report that was conducted," NBC spokesperson Hilary Smith told the magazine in October. "We fired Matt Lauer within 24 hours of learning what he did, and promptly launched a corporate investigation. Based on our investigative findings, we have swiftly taken appropriate actions to address the situation and improve workplace culture.”

Spokespersons for Philadelphia-based Comcast and the DNC did not immediately return requests for comment.

The letter was first reported by the Huffington Post.

Staff writer Julia Terruso contributed to this article.