Samira Salih al-Nuaimi was seized from her home on Sep 17 after allegedly posting messages on Facebook that were critical of the militants' destruction of religious sites in Mosul. Her Facebook page appears to have been removed since her death.
According to the United Nations Assistance Mission in Iraq, al-Nuaimi was tried in a so-called "Sharia court" for apostasy, after which she was tortured for five days before the militants sentenced her to public execution.
The militant group captured Iraq's second largest city Mosul during its rapid advance across the country's north and west in June, as Iraqi security forces melted away. The extremists now rule a vast, self-declared caliphate straddling the Syria-Iraq border in which they have imposed a harsh version of Islamic law and beheaded and massacred their opponents.
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In August, the group destroyed a number of historic landmarks in the town, including several mosques and shrines, claiming they promote apostasy.
The Gulf Center for Human Rights said yesterday that al-Nuaimi had worked on detainee rights and poverty. The Bahrain-based rights organisation said her death "is solely motivated by her peaceful and legitimate human rights work, in particular defending the civil and human rights of her fellow citizens in Mosul."