(CNN) An 800-year-old mummy, believed to be the remains of a Crusader, has been decapitated by vandals at a church in Ireland.

Thieves broke into the crypt at St. Michan's Church in Dublin and vandalized the burial site before making off with the head, the Church of Ireland said.

The mummy is thought to be a soldier who fought in the Crusades, a series of eight expeditions sanctioned by the Church between the 11th and 13th centuries, which resulted in a religious war between Christians and Muslims.

Intruders turned the body over while removing its head, and also desecrated the 400-year-old remains of a nun, according to the Church.

The vandalism, which is being investigated by police, was discovered on Monday as a guide was preparing to open the church.

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