Public prosecutors will hold the man for four days pending investigations into the death of Bishop Epiphanius, 68, whose demise rocked the Middle East's biggest Christian minority. His lawyer said he expects the detention to be extended by 15 days.
Epiphanius' body was found with head injuries in late July lying in a corridor of the monastery, according to the church.
After his death the church imposed restrictions on the monks, including disabling all of their social media accounts, according to state media.
The detained monk, identified by his monastic name of Isaiah, was defrocked a week after the incident for "strictly monastic reasons."
In the spotlightEgypt's Coptic Orthodox Church is one of the world's oldest Christian communities. As the birthplace of Christian monasticism, Egypt's deserts are home to some of the world's oldest monasteries, which have drawn monks for centuries to lead solitary ascetic lives.
The incident has thrust the church's monks into the spotlight after years of relative isolation during the monumental political changes that have shaped Egypt.
Christians in Egypt make up an estimated 10 percent of its roughly 96 million population. In recent years the church has been the victim of various attacks, largely from Islamist groups but also from more mainstream fronts.
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