Two men are being investigated over the murder of a priest in a church in northern France.

One of those arrested on suspicion of "terrorist association" on Sunday is the cousin of one assailant who slit the throat of a priest during mass last Tuesday. Both killers were shot dead by police after they held five parishioners hostage in Saint-Etienne-du-Rouvray, murdering the priest and seriously injuring one other. ISIS claimed responsibility for the attack.

Farid K, 30, is related to Abdel Malik Petitjean, and the other man under investigation, Jean-Philippe Steven J, 20, allegedly tried to travel to Syria with Petitjean in June.

Both men are being held in custody, according to the Paris prosecutor's office.

The arrests came after it emerged Petitjean was on the French terror watch list and previously been arrested for trying to travel to Syria and join terrorist forces fighting there. He had worn a tag and police had been looking for him for several days before the attack.

The other killer, Adel Kermiche, 19, was also known to the security services.

Meanwhile across France and Italy on Sunday Muslims attended Catholic mass as a gesture of solidarity after the attack.

France's Muslim council, the CFCM, urged Muslims to show "solidarity and compassion" over the killings. One Catholic woman who attended mass told Reuters: "The sentiments expressed were very, very strong. Some of them were very poignant."

A Muslim woman who came to mass with her children said: "This was basically a message of unity, aside from peace, it was really about unity."