Western intelligence officials are trying to identify Islamic State (IS) militants seen in the video that shows the beheading of US aid worker Abdul-Rahman Kassig and 18 Syrian prisoners.
Mr Kassig, 26, is the fifth Western hostage to be killed by IS.
A French prosecutor said one militant was Maxime Hauchard, 22, and another Frenchman might have been present.
A third militant was reported to be a Briton, although the father of Nasser Muthana, 20, now denies it is his son.
The Daily Mail had earlier reported Ahmed Muthana as saying "it looks like my son".
But Mr Muthana now says his son is not among the jihadists in the video.
Analysis: Frank Gardner, BBC security correspondent
The 16-minute video posted online by IS, while graphically sadistic, also reveals a wealth of detail about the perpetrators and their whereabouts. This is deliberate. IS is effectively taunting the Western-led coalition that is arranged against it, saying this is who we are, come and get us if you dare.
Intelligence agencies will be using facial recognition software to identify those involved in the mass beheading, matching their real names and origins to their adopted battle names. IS has even put a place name on the video, Dabiq in northern Syria - a place where according to Islamic hadith, an apocalyptic battle will be fought between Muslims and non-Muslims.
The one person who keeps his identity concealed is the suspected British jihadist known in the UK media as "Jihadi John". He is believed to be from London and both the FBI and MI5 almost certainly know who he is but for reasons known to them, they are not revealing it.
The IS video shows a masked man standing over a severed head, which the White House confirmed was Mr Kassig's.
He was captured by IS, which controls large parts of Syria and Iraq, in October 2013 while travelling to Deir Ezzour in eastern Syria.
His parents, Ed and Paula, from Indiana, said in a statement they were heartbroken by his death.
"We are incredibly proud of our son for living his life according to his humanitarian calling," they said. "We will work every day to keep his legacy alive as best we can."
US Secretary of State John Kerry said in Washington: "[IS] leaders assume that the world will be too intimidated to oppose them. But let us be clear: We are not intimidated."
The latest IS video also shows the beheading of 18 Syrian captives, who are identified as army officers and pilots. They are said to have been taken from Tabqa air base last August. They are mostly from the Alawite religious sect and come from Latakia and Tartous, according to UK-based monitoring group the Syrian Observatory for Human Rights.
Unlike previous videos released by IS, the latest shows the faces of many of the militants and specifies its location - Dabiq in Syria's Aleppo province.
Paris prosecutor Francois Molins identified Maxime Hauchard as appearing in the video.
Interior Minister Bernard Cazeneuve said earlier that Hauchard, who was born in 1992 and was originally from the Eure region, "had gone to Syria in August 2013 after a stay in Mauritania in 2012".
Hauchard told French TV station BFM via Skype in July that he was in Raqqa in Syria and had joined IS, adding: "The personal objective of everyone here is shahid (martyrdom). That is the greatest reward."
Hauchard's uncle told French TV on Monday: "I can't believe it was he who cut anyone's head off. It's not possible. He wouldn't hurt a fly. He was a calm and happy little boy."
Mr Molins said another Frenchman could be among the militants but that it had not yet been confirmed.
In Britain, Muthana's father, Ahmed, initially said it was his son, but then said the images were blurry and he was not sure.
He now says that although he has not seen the footage, he is able to confirm it is not his son from images he has seen.
Mr Muthana told the BBC: "It doesn't look like him, much difference. This one's got a big nose, my one has a flat nose."
Nasser Muthana appeared in an IS video in June that was aimed at recruitment.
The man in question stands to the right of another man, who is suspected to be British militant nicknamed "Jihadi John". This man is dressed in black with a balaclava, while the rest of the militants wear army fatigues.
"Jihadi John" has been shown in previous IS videos of the beheadings of the other Western hostages: Britons Alan Henning and David Haines, and US journalists James Foley and Steven Sotloff.
- Former US Army Ranger who served in Iraq in 2007
- Trained as an emergency medical technician and founded an organisation to supply refugee camps in Syria
- Snatched in October 2013 while travelling in eastern Syria for a project
- Changed his name from Peter Kassig when he converted to Islam in late 2013