AppNexus, a major provider of automated online ads for publishers, recently banned the “alt-right” news website Breitbart from using its ad-serving tool, citing violations of its rules against hate speech. But Google still feeds ads to the controversial outlet, and it appears that automated systems from Amazon, Facebook, and others might also provide ad-related services, though given their complexity it’s sometimes hard to verify.

“We would ban this as quickly as a site that has pornography and violence,” AppNexus spokesperson Josh Zeitz told Bloomberg. The ban started about five days ago, Zeitz said.

Breitbart has run stories with such headlines as “Why Equality and Diversity Departments Should Only Hire Rich, Straight White Men.” Its former executive chairman Steve Bannon was recently named US president-elect Donald Trump’s chief strategist.

Ad networks deploy trackers on publishers’ websites so they can better target individual users. While these trackers are invisible to the casual reader, they can be discovered and catalogued with free tools like the ad-blocking software Ghostery.

A Ghostery tour of Breitbart reveals dozens of trackers that either serve ads or provide analytics or other data to advertisers or the publisher. These include trackers from Google, Amazon, Facebook, and AOL.

Google’s ads, served by the AdSense network and including some major advertisers, are clearly visible on Breitbart’s website. They can be identified without using a tool like Ghostery, as they contain a blue “i” icon in the top right. Hovering your cursor over that icon reveals the text “Ads by Google.”

Screenshot of Breitbart An ad served by Google’s AdSense on Breitbart.

Others are harder to find. For instance, Ghostery shows that Breitbart has embedded a tracker for Amazon Associates, an ad service that displays products for sale at Amazon.com on a news or other kind of site. The service allows publishers to select which Amazon items they want to advertise on their sites. Publishers get a cut if a reader clicks through and buys a product.

Not all the trackers on Breitbart are about placing ads alongside its content. Some help the news site advertise itself elsewhere online. For instance the Facebook Custom Audiences tracker allows the outlet to more effectively target readers who are also Facebook users. A Breitbart reader may thus see a Breitbart ad when browsing Facebook.

Some of these advertising schemes contain terms of service that might provide grounds for barring Breitbart. For instance, Amazon Associates says users of its Product Advertising API should not “promote discrimination.” Google’s AdSense content guidelines say its ads may not be placed next to content that “advocates against an individual, group, or organization.”

A Google spokesperson said it had no comment about whether it would continue to allow AdSense ads on Breitbart. Breitbart didn’t immediately respond to a request for comment, and neither did Facebook or Amazon.

Online ads are a big—and confusing—business. The mere presence of a tracker doesn’t necessarily mean a website is currently serving ads from that ad network. For instance, Ghostery still shows an AppNexus tracker on Breitbart, despite the news site being banned for the past five days. A closer inspection of the code linked to the tracker shows that AppNexus is rejecting Breitbart’s request for ads, according Zeitz.

AppNexus’s push to clean up the ads it carries began in 2013, when it established a “zero-tolerance” policy on fraudulent or invalid activity. This included sites that published hate speech. It formed a 12-person team to audit suspicious sites at the time. Breitbart was barred after a “human audit” of its content found that it included coded or overt forms of hate speech, Zeitz told Bloomberg.

While Breitbart is being sanctioned for promoting hate speech, other publishers have been cut off from ad dollars for promoting fake news. Made-up stories circulating on social media have been blamed for influencing the outcome (paywall) of the US presidential election. Google has pledged to restrict ads placed on fake news sites, and Facebook followed suit.

Additionally, since online ads are increasingly customizable, different users see different ads based on their browsing history, location, and other factors. These customized profiles allow ads to follow a user across different websites, in a practice called retargeting. ”You go to a website and you’ll see different ads from the next guy,” says Kevin Peterson, head of technology at Pixels, a major ad network in Hong Kong. “For instance, I’m an iHerb [a seller of nutritional supplements] customer so I’ll see their ads all over Breitbart.”

With the rise of programmatic advertising, which often employs auctions of advertising space conducted by algorithms in milliseconds, there’s also the possibility of ads being served on websites that brands never intended to be associated with. It’s such a confusing state of affairs that some brands only discovered they were advertising on Breitbart after customers complained.