Cameron spoke to the press following meetings regarding the video showing Foley's beheading by a member of the Islamic State (IS) who delivered a threat in an English accent. The Federal Bureau of Investigation confirmed the video's authenticity.
"We have not identified the individual responsible on the video but from what we've seen but it looks increasingly likely that it is a British citizen. Now this is deeply shocking but we know that far too many British citizens travel to Iraq and travel to Syria and take part in extremism and violence," said Cameron.
He said that the government needs to focus on stopping nationals from joining the radical group in order to keep the group from committing further atrocities.
"What we must do is to redouble all our efforts to stop people from going, to take away the passports of those contemplating travel, to arrest and prosecute those that take part in this extremism and violence, to take extremist material off the internet and do everything we can to keep our people safe and that is what this government will do," Cameron told reporters.
Cameron left his vacation early after the video emerged to return to London to discuss the threat of IS in Iraq and Syria.
U.S. President Barack Obama called the murder "an act of violence that shocks the conscience of the entire world," during a press conference in Martha's Vineyard.
Obama said the U.S. will continue to "do what we must do to protect our people."