Investigators identifying the killer of American journalist James Foley now believe there are two masked fighters in the execution video - one who made the English speech and one who beheaded him.
CNN reported that the investigators have gathered some "disturbing clues" after studying the long English speech by the jihadist and the gruesome execution video.
They believe the jihadist who spoke in English was a different person from the one suspected of killing Foley. The English speaker appeared to be of a different height from the suspected killer, the news network reported.
It also said there was "a problem with continuity" as the knife wielded by the English speaker was different visibly, in terms of style and dimensions, from the knife left next to the victim.
In addition, the pistol of the English speaker was hoisted on the left side of his body, suggesting he was right-handed. But the suspected killer appeared to be left-handed in the video, said CNN
"The person who is essentially the mouthpiece may not have been willing to carry through with the deed or may not have been able to,'' CNN quoted Ross Patel, Director of Forensic Science at Afentis Forensics, as saying.
The report also identified the accent of the English speaker as that from "deprived, multi-ethnic areas in London".
"The speaker is using a variety of English known as multi-cultural London English. And that's a kind of melting pot accent that has emerged in recent years, particularly in deprived multi-ethnic areas in London,'' said voice forensic analyst Martin Barry.
Meantime, Fox News reported that federal investigators have identified the social media account used to post the execution video. It said the jihadist who posted the video is part of a group of Islamic extremists who are active on social media platforms and are close to the leadership of the Islamic State in Iraq and Syria (ISIS), also known as the Islamic State (IS).
ISIS frequently posts pictures of senior leadership and its "proximity" to other key militants - and a counter-terrorism source close to the matter said the data suggests the operation to kill Foley was controlled by top ISIS leaders, with Abu Omar al Baghdadi providing his blessing or tacit approval, reported Fox News.
Adel Majid Abdel Bary, a 23-year-old British-born rapper, is the leading suspect in the killing. His background as middle-class and educated is consistent with the new generation of digital jihadists, sources say, according to the report.
Many within the extremist group, which is now being closely monitored by US counterterrorism officials, are shifting their routines to become harder to trace. They are increasingly moving away from Twitter, or using peer-to-peer message-sharing systems once typical social media accounts are shuttered.
Foley, 40, was kidnapped in 2012 while covering the Syrian uprising. His captors demanded US$132.5 million (S$166 million) from his parents and political concessions from Washington.
A senior Obama administration official said last week that Islamic State leaders made a "range of requests" from the US for Foley's release, including changes in American policy, Fox News reported.