A boy reported to be 10 years old appears to shoot a Palestinian hostage in the head in the latest video posted by Islamic State militants.
The child is described in the recording as one of the "cubs of the caliphate" before shooting the 19-year-old man at close range.
Both the boy and a man in the chilling propaganda video have been identified as French citizens by an official with close ties to intelligence services.
Investigators are also looking into whether the man is related to an extremist who attacked a Jewish school in southern France in 2012, the official told The Associated Press.
The official also said another French fighter whose death in a suicide bombing was announced this week by Islamic State is a young teenager.
The man in the video speaks with a southern French accent and looks like the step-brother of Mohammed Merah, who killed seven people in attacks on a Jewish school and paratroopers in the south of France beginning on March 11, 2012 - exactly three years ago.
In the video, the man praises attacks on Jews “in your own stronghold in France” as he and the boy stand behind the man about to be killed.
In addition to the 2012 killings in Toulouse, Jews in a kosher supermarket were among the targets of three days of terror in the Paris area this year that left 20 people dead, including the three gunmen.
“Here are the young lions of the caliphate,” the man says in the video. Soon afterwards, the child is shown shooting the man in the head.
French government spokesman Stephane Le Foll described the video as a “dreadful” killing, but refused to comment on the nationalities or identities of the man and boy.
The French official with intelligence ties also confirmed the nationality of a French fighter whose death was announced this week by the Islamic State group.
The group said Abu Ali al-Firansi died in a suicide bombing in Tikrit. The French official described him as a young teen.
About 1,400 people, including entire families, have left France to join extremists in Syria and Iraq, and many have returned.
Security officials fear some will arrive with honed skills as fighters, and with passports that allow free travel.
“Nearly 90 French citizens have died there, weapons in hand to fight against our values,” French Prime Minister Manuel Valls told France’s iTele on Sunday.
About 3,000 Europeans are part of the fight, Mr Valls said, adding: “There could be 5,000 before the summer and without a doubt 10,000 by the end of the year.”
The man killed in the video has been identified as Palestinian Mohamed Said Ismail Musalam.
His father Said Musalam says his 19-year-old son was tricked into joining the militants.
He says IS lured him with promises of women, money and cars.
He says Mohamed contacted the family from Syria, saying he wanted to come home.