In the Islamic State group’s Iraqi stronghold of Mosul, some Christian churches are no longer places of worship but rather sites of arbitrary detention and brutal treatment of prisoners. Mosul is home to some of the oldest churches in the region, but following the militant insurgency, at least two have been turned into prisons and are at risk of being destroyed, according to local Christians.

Several prisoners were sent to the Chaldean Church of the Immaculate Conception on Tuesday, and St. George’s Monastery is also reportedly being used as a prison for female detainees, according to a report from Fides News Agency. Last week, reports circulated that militants blew up part of St. George’s Monastery. The destruction of St. George's and/or the Church of the Immaculate Conception would be a significant blow to the local Christian community.

It is unclear whether the group also known as ISIS or ISIL are utilizing the churches as an insult to Christians or if they're simply convenient. Both buildings are below street level and feature heavy stone walls dating back to the 10th century, making them ideal places to keep detainees. 

It is still unknown who is imprisoned in the churches and what kind of treatment they face, but many suspect torture. Detainees who were brought to the Chaldean church on Tuesday were handcuffed and blindfolded, according to Fides.

A little over a decade ago, Mosul was home to around 30,000 Christians -- the largest population of Assyrian Christians in Iraq outside of Kurdistan -- but crackdowns by Iraq’s then-dictator Saddam Hussein significantly lowered that number, according to the New York Times. Thousands of Christians, however, still lived in Mosul when ISIS seized the northern Iraqi province and declared a caliphate in June. Nearly a month later, militants issued a statement giving remaining Christians 24 hours to flee. An estimated 20,000 Christians left Mosul within 45 days of ISIS' occupation, according to a report from the United Nations refugee agency.

“We are not thinking of going back to Mosul; we have left homes with our memories,” Omar, a former Christian resident of Mosul who did not give his last name, recently told the New York Times. “It is a sad time for Christians.”

Militants then began to destroy sacred Christian monuments in the city. In July, the group detonated explosives all around the Mosque of the Prophet Sheeth (Seth) and the Mosque of the Prophet Younis (Jonah), which is said to house Jonah's tomb, according to al Arabiya.

Mosul is a major target in the U.S.-led coalition anti-ISIS airstrikes, and remaining Christians fear that whatever sacred spaces ISIS spares will be destroyed as collateral damage. "Among the concerns that plague us, there is also that of those who fear that a possible military offensive for the liberation of Mosul could inevitably lead to consider churches as targets to hit, since they have become logistic bases of the jihadists,” Rebwar Audish Basa, procurator of the Antonian Order of St. Ormizda of the Chaldeans, told Fides. “And, of course, the destruction of old churches would be an irreparable damage and loss."

St. George’s Monastery was built in approximately the 17th century and is one of the oldest churches in Mosul. The Church of Immaculate Conception was the first Chaldean Catholic Church in Mosul and represented a community of around 500,000 Assyrian Christians at one time. It dates back to at least the 10th century.