The FBI has confirmed that the propaganda video showing the murder of US journalist James Foley is authentic.
American and British intelligence agencies are trying to identify the killer, who speaks with a British accent.
Islamic State says the killing was in revenge for American air strikes in Iraq.
James Foley who was 40 had reported extensively from the Middle East and other world trouble spots, working for the news website Globalpost among others.
He had been detained by militants during a spell in Libya.
There have been calls on social media for people not to share the images of his death but remember his life and work.