SAINT-ETIENNE-DU-ROUVRAY, France—Two followers of Islamic State killed a priest while he was celebrating Roman Catholic Mass Tuesday, the first time a church has been attacked amid a wave of terrorist violence rocking Western Europe.

French prosecutors identified one of the attackers as Adel Kermiche, a 19-year-old French national who served 10 months in prison for twice trying to travel to the battlefields of Syria. French judges ordered him released from prison in March 2016 on condition he wear an electronic monitoring...