The top trending video on YouTube Wednesday morning advanced a burgeoning conspiracy theory that a survivor of last week's shooting at a Florida high school was a "crisis actor."
By 11 a.m. ET, the video had been removed from YouTube — but could still be viewed on other sites.
"This video should never have appeared in Trending. Because the video contained footage from an authoritative news source, our system misclassified it. As soon as we became aware of the video, we removed it from Trending and from YouTube for violating our policies. We are working to improve our systems moving forward," a YouTube spokesperson told CNBC.
Mary deBree, Head of Content Policy at Facebook, also said in statement that "we are removing this content from Facebook."
Similar videos and links also flooded Facebook and Google on Wednesday, some garnering tens of thousands of shares.
Facebook and Google searches for "crisis actor" or the student's name, "David Hogg," return a mix of links: Some legitimate news sources citing Hogg's denial of the claim, and some spreading the false report.
A search for Hogg's name on YouTube also revealed that some of those false reports could still be found on the site Wednesday afternoon.
The term "crisis actor" is sometimes defined as an actor who goes from crisis to crisis to advance a certain ideology.
CNBC searched "David Hogg" in Google News just before noon Wednesday, and found false reports listed as "Related Coverage" alongside legitimate reports.
It shows that major tech companies are still unable to keep false news items off their platforms, despite the avalanche of criticism they've been hit with since the 2016 election.
Facebook and Google faced similar challenges in November when a gunman opened fire on the crowd at a Las Vegas concert and false reports appeared on the sites .
The companies at the time said they work to curtail the spread of misinformation.
— CNBC's James Thorne contributed to this report.