The US Justice Department is conducting a criminal investigation of the death of American journalist James Foley, Attorney General Eric Holder said on Thursday.
Foley was beheaded by the Islamist militant group Islamic State, an act shown in a video released on Tuesday in which the group called for the United States to end its air strikes in Iraq. President Barack Obama responded that the United States would be relentless in fighting the organization despite the killing.
A European security official said Foley's executioner, a masked man in black who used a knife to behead Foley, spoke with an accent from London or near London. Some officials suggested it was possible the man was originally an immigrant to the United Kingdom.
A US official said American investigators were urgently attempting to "identify and capture" the man in the video. The official did not say who would take the lead in trying to capture him once he is identified.
US and European officials said efforts were underway to match the voiceprint of the man in the video with recordings of known individuals.
In addition to the FBI and Justice Department, British government agencies, including the counter-intelligence agency known as MI-5, were working on the case. MI-5 has devoted considerable effort to identify and trace Britons fighting with militants in Iraq and Syria.
British authorities have estimated that as many as 500 U.K. citizens have traveled to Syria to fight with the Islamic State or with rival Jamat al Nusrah militants. A European official said photos previously posted on militants' social media feeds had pictured men identified as Britons displaying multiple severed heads.