Gunman Elliot Rodger's videos removed by YouTube
The Google-owned YouTube has removed videos posted by Elliot Rodger, the gunman who killed six students in California last week.
The site said the clips violated its terms of service regarding videos that threatened violence.
A spokeswoman said the videos were "against YouTube's guidelines" and were removed when flagged.
But experts warned that removing them might accelerate their spread around the internet.
Before killing three of his housemates, and then going on to shoot at random from his BMW, Rodger posted videos on the video-sharing site describing his plans.
In one, filmed in his car, he said: "Tomorrow is the day of retribution. The day in which I will have my revenge against humanity, against all of you."
The incident has led some to question whether technology can be used to flag disturbing content online as a way of stopping similar events.
Google, like other companies that offer media-sharing services, has developed sophisticated methods to detect and deal with videos that contain copyrighted content, such as music.
But to apply the same techniques to disturbing human behaviour was a challenge both enormous and complex, said experimental psychologist Andrew Przybylski, from the Oxford Internet Institute.
"If there was an algorithm to identify people who were not functioning well, that would at first be interesting to the mental health community," he said.
"People are asking Google for a technical solution for a social issue."
By trying to monitor and act on videos that caused concern there will be a lot of false alarms, he added.
"A lot of kids, when they're developing their identity, will say extreme things, but that doesn't mean that's a threat."
Despite Google's actions to remove the content, the videos have been reposted around the internet - an unavoidable outcome, Dr Przybylski said.
"The act of actually trying to hide the material actually will lead people to invest more value in it," he said.
Videos that are removed in this way suffer from the so-called "Streisand effect" - the theory that by trying to block something, the actual result is a far wider dissemination.
Elsewhere online, a page on Facebook that sought to "pay tribute" to Rodger was taken down after pressure from users.
The social network had initially said it did not consider the site to be a breach of its terms of service, but went on to remove the page on Monday evening.
The Guardian newspaper speculated that the page may have been intentionally set up as an act of provocation, rather than a genuine tribute.
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