One of the men suspected of killing a priest at a church in northern France was being monitored by police and was wearing a surveillance tag at the time of the attack, officials say.
Prosecutor Francois Molins said Adel Kermiche, 19, was arrested twice last year trying to reach Syria, BBC reported.
Kermiche and a fellow attacker stormed the church in Saint-Etienne-du-Rouvray, a suburb of Rouen, during morning Mass.
They slit the throat of the elderly priest before being killed by police.
One of four people taken hostage—said to be an elderly parishioner-suffered severe knife wounds, Molins said.
Molins said the two attackers had been carrying a "fake explosive device covered in aluminum foil" along with handheld weapons when they entered the Catholic church.
As they targeted Father Jacques Hamel, in his 80s, some of the congregation were able to escape and alert the police, who sent in a team specialized in dealing with hostage situations.
"Three of the hostages were used as human shields to block the police from entering the church," Molins said.
When they were eventually let go, the two attackers followed them out of the church shouting "Allahu Akbar" before being killed, Molins said.
Earlier, the self-styled Islamic State terrorist group claimed its "soldiers" carried out the attack.
