The new revelations come from a CNN investigation, which found that companies such as Amazon, Adidas, Facebook, Mozilla, Netflix, Nordstrom and Under Armour were affected. Five U.S. government agencies also had their ads placed on extremist videos, as did religious organizations and some film studios.

Many of the companies said they did not know their ads had been placed on the videos and were taking action to figure out why they were placed there. When companies advertise with YouTube, they can target their ads based on user behavior and demographics. They can also block specific channels and choose a “sensitive subject exclusion” filter, according to CNN, which should prevent ads from appearing on certain channels.

YouTube said it wants to work with advertisers to control inappropriate content, but it did not address how the latest issues occurred.

“We have partnered with our advertisers to make significant changes to how we approach monetization on YouTube with stricter policies, better controls and greater transparency,” a spokeswoman for the company told CNN in a statement. “When we find that ads mistakenly ran against content that doesn’t comply with our policies, we immediately remove those ads. We know that even when videos meet our advertiser friendly guidelines, not all videos will be appropriate for all brands. But we are committed to working with our advertisers and getting this right.”

This is not the first time companies have found their ads placed on extremist content. Last year, hundreds of companies cut back on their YouTube ad-buys after they learned their advertisements were being displayed on controversial content. But after that incident and others like it, advertisers returned to the platform.

YouTube’s massive user base and the billion hours of video watcged each day makes it a crucial platform for advertisers.

This time, some companies, such as Under Armour, are again saying they will pause advertising on YouTube until they can determine what happened.

“We have strong values-led guidelines in place and are working with YouTube to understand how this could have slipped through the guardrails,” a spokesperson for Under Armour told CNN. “We take these matters very seriously and are working to rectify this immediately.”

YouTube has taken other steps this year to address the issue of extremist content on its site. In January it changed the rules to restrict which videomakers could earn money from ads and said it would have people review all its most popular videos.