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.