A friend of one of the two men who killed a French priest in Normandy was detained only days before the attack took place, reports say.
The 17-year-old tried to travel to Syria last year with Adel Kermiche, French and Swiss media say.
The unnamed teenager was reportedly arrested in Geneva earlier this month and handed over to French authorities.
Days later, on Tuesday, his friend Kermiche went on to kill Father Jacques Hamel in his church near Rouen.
The news of the detention of a close friend of Kermiche, who pledged allegiance to the Islamic State group in a video, comes at a time when French intelligence services are under close scrutiny.
Reports in Le Parisien and the Tribune de Geneve newspapers said it was not known why the 17-year-old, from northern France, went to Geneva earlier this month.
He had already tried to fly with Kermiche to Syria via Switzerland last year. Both were turned back from Turkey.
In the latest development, Swiss security services picked up the teenager, and another friend, on 20 July and handed them over to French authorities.
A source told AFP news agency that “nothing suggests he was in any way implicated” in the church attack on 26 July.