Fr Giovanni Martire Savina from Santa Maria delle Grazie alle Fornaci, a church near the Vatican, said he's never seen anything like it and initially thought it was a bomb.
"Until a few years ago every now and then someone would leave substantial monetary offers, even €1,000, but never this much," he told Il Messaggero.
"Certainly, such a sum would be useful for the church, but not to the detriment of someone else."
Rome police were called on the church on Wednesday and said officers also found €50 notes hidden in the confessional box under the priest's chair.
An investigator said the money may have been stashed by someone who felt guilty for obtaining the cash illegally or by just an innocent generous donor.
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