SAN JOSE, Calif. — Silicon Valley firms were prepared this week to quickly block video of an Islamic State militant beheading an American journalist after a previous video by the same group showing the death of James Foley ricocheted through social networks in what was seen by some as a propaganda coup for the extremists.

The video Tuesday showing the beheading of American journalist Steven Sotloff was first uploaded onto a different website and quickly deleted when copied onto YouTube, slowing the spread of posts linking to it, said a Silicon Valley insider, speaking on condition of anonymity because of the sensitivity of the topic.

Facebook, Twitter, YouTube and others all have policies banning terms of service regarding images of gratuitous violence or that incite hatred. But grisly images, once viral, are hard to restrain.

“It’s been very interesting, with this second beheading, how very little of those images have been passed around,” said Family Online Safety Institute CEO Stephen Balkam, who serves on Facebook’s safety advisory board. “It’s very difficult to find them unless you know of some darker places on the Web.”

When Tuesday’s beheading video of Sotloff was launched so soon after Foley’s death, “platforms were better prepared for it this time around,” the Silicon Valley insider said. Social media firms are trying to force out the Islamic State group “platform by platform,” the tech official said.

YouTube in a statement said it has “clear policies prohibiting content intended to incite violence, and we remove videos violating these policies when flagged by our users.”