After Mark Zucker, founder and CEO of social media giant Facebook, said in an interview that some Holocaust deniers “aren’t intentionally getting it wrong,” and therefore Facebook would not remove their posts, the Anti-Defamation League is challenging Facebook’s position.

The ADL wants Facebook to take a stronger stand against Holocaust denial because “at its core” it is hate and anti-Semitism.

In an op-ed in the New York Daily News, headlined “Holocaust denial is a form of hate,” ADL CEO Jonathan Greenblatt wrote: “Facebook has a moral and ethical obligation not to allow its dissemination. ADL will continue to challenge Facebook on this position and call on them to regard Holocaust denial as a violation of their community guidelines.”

He also spoke out in an interview with CBS News —watch and listen here.

Zuckerberg’s comments drew widespread condemnation on social media and in the press, prompting him to clarify his statement hours later.

“I personally find Holocaust denial deeply offensive, and I absolutely didn’t intend to defend the intent of people who deny that,” he wrote in an email published on the tech news website Recode.

Mark Zuckerberg’s sister Randi, weighing in on her brother’s controversial comments about Holocaust deniers, forcefully denounced such people and “their hateful, disgusting rhetoric.” But she said banning them from social media “will not make them go away,” and she emphasized the importance of a healthy debate over the role tech companies should play in policing content.

Randi Zuckerberg, who has long worked with many Jewish community organizations, spoke out  last Thursday in a statement she provided exclusively to CNNMoney. Her comments came one day after her brother drew criticism for telling Recode’s Kara Swisher that some Holocaust deniers “aren’t intentionally getting it wrong,” and so Facebook would not remove their posts.

“As a leader in the Jewish community, and someone who has worked at the ground floor of social media, I felt a responsibility to weigh in,” she wrote.

Randi Zuckerberg, an early Facebook employee and successful entrepreneur in her own right, said her brother “could have chosen his words differently,” but she applauded him for “navigating this incredibly difficult new world where the notion of free speech is constantly changing.” And she lamented that a platform that has connected Jewish organizations and united people around the world also can be used as a weapon against them.

“Unfortunately, when we give a voice to everyone, we give it to people who use that voice for good and to people who abuse that voice,” she wrote. “Organizations doing impactful work now have more powerful tools than ever before, yet the nasty dark underbelly that exists right beneath the surface has access to those exact same tools.”