Islamic State has released a video of the two men it says murdered a French priest while he was leading morning prayers.
The video shows the two men swearing allegiance to the IS leader.
Police have now identified both teenagers IS says were responsible for the attack, one was being electronically monitored having tried twice to reach Syria.
French President Francois Hollande called the attack a cowardly assassination and called for unity in the wake of the incident.
"All people feel affected so we must have cohesion. ... No one can divide us," he said. "Terrorists will not give up on anything until we stop them."
The killers had taken 86-year-old Catholic priest Jacques Hamel hostage as well as three other people, including two nuns.
Police were alerted to the incident but could not enter the building because of the hostage situation.
One of the men was wearing a fake explosives belt, the other carried a kitchen timer and a fake bomb.
The men slit the priest's throat and stabbed him in the chest during what was described by witnesses as a religious oration in Arabic.
One of the men shouted "Allahu Akbar," "God is the greatest" in Arabic as they left the church.
Police then "neutralised" the situation.
France is in a state of emergency following a string of terrorist attacks.