A criminal investigation has been launched into the beheading of American journalist James Foley, US Attorney General Eric Holder said.
A video was released of the reporter being killed by a militant from the Islamic State (IS) in which the group says it is acting in retaliation for US airstrikesin Iraq.
Counter-terrorism police in the UK have already begun efforts to identify the black-clad man beheading the 40-year-old.
Although his face is covered, he speaks with a British accent.
People previously held hostage by IS have suggested he may be a jihadistknown as “John” who was part of a group guarding captives in Syria.
A former hostage, who was held for a year in the Syrian town of Raqqa, told The Guardian the killer was the ringleader of a trio of UK-born extremists the captives nicknamed “The Beatles” because of their nationality.
IS pressed the American government to provide a multi-million-dollar ransom for Mr Foley’s release, according to The New York Times.
The US – unlike several European countries that have given millions to the terror group to spare their citizens – refused to pay, the paper said.
American airstrikeshave continued against IS forces in northern Iraq, despite the group threatening to kill a second US captive if attacks go on.
US Navy fighters and drones provided air cover to Iraqi and Kurdish forces trying to retake and maintain control of Mosul Dam.
The US military has carried out 90 air strikes in Iraq since August 8, with 57 of them in support of Iraqi government forces near the dam.
Meanwhile, Interpol has called for a globally co-ordinated push to stop the tide of international fighters joining IS, also known as Isis.
It did not give any specific recommendations, but said it is particularly concerned that Mr Foley’s killer may be British.
“(This highlights) the need for a multilateral response against the terror threat posed by radicalised transnational fighters travelling to conflict zones in the Middle East,” said Interpol Secretary-General Ronald Noble.
More than 1,000 radicals from Europe have joined Islamist militants in Syria and Iraq, and Interpol has long warned of the threat such fighters pose.
European governments fear they could stage attacks when they get home and have introduced new anti-terrorism measures to try to catch them or stop them leaving in the first place.
Earlier, a US journalist and Muslim convert – who has experience of Western jihadi fighters in Syria – told Sky News the UK Government’s approach to the issue is completely wrong.
Bilal said: “Isis realises that by using a Western voice to push the narrative for them, it would get a coverage that they would not have gotten, had that been a Syrian voice or an Iraqi voice.”
He added: “They operate very much like a gang-like mentality. There are some Brits that are there that wanted out … but they don’t have any place to go. Where are they going to go to exactly?
“The issue is that if they were to leave the ranks of Isis (are) they going to trade that for a UK jail cell?
“I think that the British government is totally clueless in terms of how to deal with this issue … and despite the education that a lot of their leaders have, they appear as if they are totally perplexed.”