The site, which has gained a large following in recent months, partly because of its free-speech ethos, is increasingly home to Holocaust denial, antisemitism, and racism, the ADL said.

High-profile Parler users include former Ku Klux Klan leader David Duke, Alex Jones’ conspiracy website Infowars, and anti-immigration site VDARE.

“These are the kinds of individuals and organizations whose toxicity is well-known and whose racism and anti-Semitism isn’t very well disguised,” said Jonathan Greenblatt, ADL’s CEO. 

Parler did not respond to a request by Fortune for comment.

While giving a home to extremism isn’t Parler deliberate strategy, the ADL said, its lax policing gives a megaphone to extremists to spew hate without consequence. That commentary exacerbates the nation’s already existing political polarization.

Amid that tumult, Parler has become one of the most downloaded apps for mobile devices. Many users have flocked to the service because of what they consider to be unfair crackdowns by Facebook and Twitter over hate speech and misinformation.

Following the presidential election, Twitter and Facebook slapped warning labels on some of President Trump’s posts that included unsubstantiated claims of voter fraud. As a result, Republican heavyweights like Sen. Ted Cruz, Fox News political commentator Sean Hannity, and Trump’s son, Eric, have encouraged their social media followers to join Parler. 

Because Parler relies on users to police its service, Greenblatt is worried about unchecked hate speech. The ADL cited a recent post on Parler by fringe website BiChute that called the Holocaust gassings a “profitable hoax.” Conservatives and extremists are currently using Parler to promote pro-Trump events like the Million MAGA March and #StopTheSteal rallies.

For years, the ADL has worked with Twitter and Facebook to help them better understand how their services contribute to hate. The ADL contacted Parler’s chief policy officer, Amy Peikoff, on Wednesday, but it did not receive a response, the organization said.

In the meantime, the ADL said it would continue to monitor Parler and will “call out hate when we see it.”

“Ultimately, that’s the goal—to make sure these digital spaces are safe and secure,” Greenblatt said. The services should “allow for diversity of ideas but that they don’t do it while degrading Jews, African-Americans, or other minorities—that they don’t demonize people for how they pray or who they love.”