The cousin of one of the killers, Farid K, has been arrested on suspicion of 'terrorist association'.
He is related to Abdel Malik Petitjean who was killed by French police.
Jean-Philippe Steven J, 20, was also put under formal investigation for allegedly attempting to travel to Syria in June with Adel Petitjean.
Petitjean and his fellow attacker, Adel Kermiche, both 19, were shot dead by police.
On Sunday, Muslims attended Catholic Mass across France on in an act of unity.
They remembered Fr Jacques Hamel, 86, who was killed on 26th July when Petitjean and Kermiche stormed his church in Saint-Etienne-Du-Rouvray while he was conducting Mass with parishioners.
It has also emerged that the Muslim community have refused to bury one of the murderers, Adel Kermiche, because of his actions.
