Three days before the attack on Tuesday, authorities found a video showing one of the assailants, Abdel Malik Petitjean, pledging allegiance to the Islamic State armed group and speaking of “a violent action”.
A 19-year-old man was charged with “criminal conspiracy with terrorists” and locked up over the video on a mobile phone police discovered at his home, a judicial source said.
Three other people were still being held by authorities for questioning.
Meanwhile, Muslim and Christian groups will hold vigils on Saturday for Jacques Hamel, the French priest who was murdered by jihadists.
In a bid to forge togetherness between the communities, a regional Muslim council has planned a "brotherhood march" in the southeastern city of Lyon.
A church in Bordeaux said it would hold a non-denominational vigil for the 85-year-old Hamel, who was killed by attackers, as he was celebrating mass in a church in a church.
- with AFP