When alerted to the ads promoting untrue stories on their sites, Snopes and PolitiFact said there was little they could do. Google’s AdSense, which is used by web publishers to sell display advertising on their sites, works through automated tools. Often, advertisers are unsure where their ads are running — sometimes next to inappropriate or offensive content — and site owners don’t know which ads will appear on their pages.

Vinny Green, Snopes’s co-owner and vice president, said it had tried to filter out misleading advertisements from the 150 million ads it displayed on its site last month. But that goes only so far.

“We have little direct oversight or control over what is being done to filter out fake news ads being served on our site,” he said in an email. He added that the online ad ecosystem was complicit in disseminating and profiting off of misinformation, and that “these ad quality problems are systemic.”

Aaron Sharockman, executive director of PolitiFact, said it was working with Google to remove the “questionable text ads” from its site.

“The revenue those advertisements provide is critical to funding a website like ours, but it’s equally important that we do everything we can to make sure the advertisements appearing on our site are not deceptive or intentionally misleading,” he said.

Google, which sells more online advertising than any other technology firm, has struggled to prevent fraudulent websites from making money off the spread of false stories. Earlier this year, it touted its efforts to crack down on misinformation sites by kicking 340 websites and 200 publishers off its AdSense platform. Most of those publishers had created websites to peddle eye-catching but untrue political stories, and loaded up the pages to get a cut of advertising revenue from Google.