Among them is American Austin Tice, a freelance journalist who was contributing articles to The Washington Post. Tice disappeared in Syria in August 2012. There has been no word of or from him since his abduction.

Searching for clues

U.S. and British counterterrorism analysts are examining every frame and piece of audio of the execution video for clues about where it took place and who the executioner is, U.S. officials, speaking on condition of anonymity, told CNN.

The voice in the video seems to have a British accent so they're trying to match any individuals known to the British government who may have gone to Syria to fight in that nation's civil war.

The analysts are looking at clothing, climate, terrain, language and wording and whether there are any National Security Agency or UK phone intercepts matching the voice, the officials said.

Foley's killing recalled the murder of Daniel Pearl, The Wall Street Journal correspondent who was kidnapped while reporting in Pakistan in January 2002. His killing was captured on video and posted online by al Qaeda.

Pearl's mother, Ruth Pearl, responded to Foley's death with a tweet posted by the Daniel Pearl Foundation Twitter account that reads: "Our hearts go out to the family of journalist James Foley. We know the horror they are going through."

Foley's death also harkened to the videotaped beheadings of Americans Nicholas Berg, Eugene Armstrong and Jack Hensley carried out by al Qaeda during the height of the Iraq War.