The Isis leader Abu Bakr al-Baghdadi viewed women held captive at a Syrian house as his private property, and forced himself upon a number of them, including the American hostage Kayla Mueller.

As systematic rape becomes an increasingly powerful recruiting tool for Isis, the full story of the ordeal faced by the aid worker killed in captivity is revealed by a 14-year-old Yazidi girl who was captured by Isis in Iraq, last August. Her version has been corroborated by US officials.

The girl says she spent two months in captivity with Ms Mueller before escaping from the Syrian town of al-Shadadiya in October 2014.

After originally being held in the Iraqi city of Mosul for little under a month, the 14-year-old and four other young Yazidi girls had, by late August of 2014, met Ms Mueller, who would have turned 27 this Friday. They were then transferred to al-Shadadiya. According to the Yazidi girl, Ms Mueller had learnt basic Arabic while in captivity and went by the name of “Kayla Carol”. The last name derived from her father’s name – which in the Arab world serves as a family name – which is Carl.

Abu Bakr al-Baghdadi, delivering a sermon at a mosque in Iraq (AP)

Ms Mueller had been kidnapped in Syria in August 2013 while leaving a Spanish Médecins Sans Frontières hospital in Aleppo. Isis had demanded a ransom of €5m for her release.

Ms Mueller demonstrated an interest in aid and human rights work during her high school years, as a member of the Save Darfur Coalition. She had travelled to the Turkish-Syrian border in December 2012 to work with the humanitarian organisation Support to Life, assisting refugees. In 2013, she spoke about her experiences to the Prescott Kiwanis Club in Arizona, a volunteer group of which her father is a member. “Syrians are dying by the thousands, and they’re fighting just to talk about the rights we have,” she said.

Kayla Mueller was kidnapped in Syria in August 2013 (AP)

As a captive in al-Shadadiya, Ms Mueller was often visited by the Isis leader, say the Yazidi girls, who cooked and cleaned in the house. Rape was a “reward” for Isis victories. And, according to those held captive, many Yazidi women passed through the house on their way to being given as “presents” or sold to Isis fighters. The Yazidi girls say the sex trade was organised by Umm Sayyaf, the Iraqi wife of Abu Sayyaf, a Tunisian Isis financier. She was captured in Syria in May by US Special Forces and has reportedly been co-operating with military interrogators.

Last week the White House announced that Umm Sayyaf would be prosecuted in Kurdish Iraq and would be “held accountable for her crimes”.

In pictures: The rise of Isis Show all 74 1 /74 In pictures: The rise of Isis In pictures: The rise of Isis Isis fighters Fighters of the Islamic State wave the group's flag from a damaged display of a government fighter jet following the battle for the Tabqa air base, in Raqqa, Syria AP In pictures: The rise of Isis Isis fighters Fighters from Islamic State group sit on their tank during a parade in Raqqa, Syria AP In pictures: The rise of Isis Isis fighters Fighters from the Islamic State group pray at the Tabqa air base after capturing it from the Syrian government in Raqqa, Syria AP In pictures: The rise of Isis Isis fighters Fighters from extremist Islamic State group parade in Raqqa, Syria AP In pictures: The rise of Isis Isis kidnapping A video uploaded to social networks shows men in underwear being marched barefoot along a desert road before being allegedly executed by Isis Getty Images In pictures: The rise of Isis Isis kidnapping Haruna Yukawa after his capture by Isis In pictures: The rise of Isis Isis kidnapping Khalinda Sharaf Ajour, a Yazidi, says two of her daughters were captured by Isis militants Washington Post In pictures: The rise of Isis Isis fighters Spokesperson for Isis Vice News via Youtube In pictures: The rise of Isis A pro-Isis leaflet A pro-Isis leaflet handed out on Oxford Street In London Ghaffar Hussain In pictures: The rise of Isis Isis fighters Isis Jihadists burn their passports In pictures: The rise of Isis Isis controls Syrian Aid A man collecting aid administered by Isis in Syria In pictures: The rise of Isis Isis controls Syrian Aid A woman collecting aid administered by Isis in Syria In pictures: The rise of Isis Isis controls Syrian Aid Local civilians queue for aid administered by Isis. Since it declared a caliphate the group has increasingly been delivering services such as healthcare, and distributing aid and free fuel In pictures: The rise of Isis Iraq crisis Iraqi security forces detain men suspected of being militants of the Isis group in Diyala province In pictures: The rise of Isis Iraq crisis Mourners carry the coffin of a Shi'ite volunteer from the brigades of peace, who joined the Iraqi army and was killed during clashes with militants of the Isis group in Samarra, during his funeral in Najaf In pictures: The rise of Isis Iraqi refugees An Iraqi Shiite Turkmen family fleeing the violence in the Iraqi city of Tal Afar, west of Mosul, arrives at a refugee camp on the outskirts of Arbil, in Iraq's Kurdistan region In pictures: The rise of Isis Isis leader Abu Bakr al-Baghdadi A photograph made from a video by the jihadist affiliated group Furqan Media via their twitter account allegedly showing Isis leader Abu Bakr al-Baghdadi delivering a sermon during Friday prayers at a mosque in Mosul. Abu Bakr al-Baghdadi declared an Islamist caliphate in the territory under the group's control in Iraq and Syria In pictures: The rise of Isis Islamic extremists destroying mosques in Iraq Shiite's Al-Qubba Husseiniya mosque explodes in Mosul In pictures: The rise of Isis Islamic extremists destroying mosques in Iraq Smoke and debris go up in the air as Shiite's Al-Qubba Husseiniya mosque explodes in Mosul. Images posted online show that Islamic extremists have destroyed at least 10 ancient shrines and Shiite mosques in territory - the city of Mosul and the town of Tal Afar - they have seized in northern Iraq in recent weeks In pictures: The rise of Isis Islamic extremists destroying mosques in Iraq A bulldozer destroys Sunni's Ahmed al-Rifai shrine and tomb in Mahlabiya district outside of Tal Afar In pictures: The rise of Isis Iraq crisis Iraqi security forces celebrate after clashes with followers of Shiite cleric Mahmoud al-Sarkhi, in front of his home in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad In pictures: The rise of Isis Iraq crisis Iraqi security forces arrest a follower of Shiite cleric Mahmoud al-Sarkhi after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad In pictures: The rise of Isis Iraq crisis Iraqi security forces arrest a follower of Shiite cleric Mahmoud al-Sarkhi at his home after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad In pictures: The rise of Isis Iraq crisis Iraqi security forces arrest a follower of Shiite cleric Mahmoud al-Sarkhi after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad In pictures: The rise of Isis Iraq crisis A vehicle burns in front of a home of a follower of Shiite cleric Mahmoud al-Sarkhi after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad In pictures: The rise of Isis Iraqi refugees An Iraqi woman holds her exhausted son as over 1000 Iraqis who have fled fighting in and around the city of Mosul and Tal Afar wait at a Kurdish checkpoint in the hopes of entering a temporary displacement camp in Khazair In pictures: The rise of Isis Iraqi refugees Displaced Iraqi women hold pots as they queue to receive food during the first day of the Islamic holy month of Ramadan, at an encampment for displaced Iraqis who fled from Mosul and other towns, in the Khazer area outside Irbil, north Iraq In pictures: The rise of Isis Isis fighters in Syria A militant Islamist fighter waving a flag, cheers as he takes part in a military parade along the streets of Syria's northern Raqqa. The fighters held the parade to celebrate their declaration of an Islamic "caliphate" after the group captured territory in neighbouring Iraq In pictures: The rise of Isis Isis fighters in Syria Isis fighters wave flags as they take part in a military parade along the streets of Syria's northern Raqqa province Reuters In pictures: The rise of Isis Isis fighters in Syria Isis fighters travel in a vehicle as they take part in a military parade along the streets of Syria's northern Raqqa province In pictures: The rise of Isis Isis fighters in Syria Fighters from the Isis group during a parade with a missile in Raqqa, Syria. Militants from an al-Qaida splinter group held a military parade in their stronghold in northeastern Syria, displaying U.S.-made Humvees, heavy machine guns, and missiles captured from the Iraqi army for the first time since taking over large parts of the Iraq-Syria border In pictures: The rise of Isis Isis fighters in Syria Isis fighters during a parade in Raqqa, Syria In pictures: The rise of Isis Isis fighters in Syria Fighters from the Isis group during a parade in Raqqa, Syria. Militants from the splinter group held a military parade in their stronghold in northeastern Syria, displaying U.S.-made Humvees, heavy machine guns, and missiles captured from the Iraqi army for the first time since taking over large parts of the Iraq-Syria border In pictures: The rise of Isis Isis fighters in Syria Isis fighters hold a military parade in their stronghold in northeastern Syria In pictures: The rise of Isis Isis fighters in Syria Isis fighters during a parade in Raqqa, Syria In pictures: The rise of Isis Isis fighters in Syria A member loyal to the Isis waves an Isis flag in Raqqa In pictures: The rise of Isis Iraq crisis Iraqi anti-government gunmen from Sunni tribes in the western Anbar province march during a protest in Ramadi, west of Baghdad. The United Nations warned that Iraq is at a "crossroads" and appealed for restraint, as a bloody four-day wave of violence killed 195 people. 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Iraqi forces pressed a campaign to retake militant-held Tikrit, clashing with jihadist-led Sunni militants nearby and pounding positions inside the city with air strikes in their biggest counter-offensive so far In pictures: The rise of Isis A police station building destroyed by Isis fighters An exterior view of a police station building destroyed by gunmen in Mosul city, northern Iraq. Iraq's new parliament is expected to convene to start the process of setting up a new government, despite deepening political rifts and an ongoing Islamist-led insurgency. Iraqi President Jalal Talabani issued a decree inviting the new House of Representatives to meet and form a new government In pictures: The rise of Isis Isis fighters in Iraq Smoke billows from an area controlled by the Isis between the Iraqi towns of Naojul and Tuz Khurmatu, both located north of the capital Baghdad, as Iraqi Kurdish Peshmerga forces take part in an operation to repel the Sunni militants In pictures: The rise of Isis Iraqi refugees An elderly Iraqi woman is helped into a temporary displacement camp for Iraqis caught-up in the fighting in and around the city of Mosul in Khazair In pictures: The rise of Isis Iraqi refugees An Iraqi Christian woman fleeing the violence in the village of Qaraqush, about 30 kms east of the northern province of Nineveh, cries upon her arrival at a community center in the Kurdish city of Arbil in Iraq's autonomous Kurdistan region In pictures: The rise of Isis Iraqi refugees An Iraqi woman, who fled with her family from the northern city of Mosul, prays with a copy of the Quran AP In pictures: The rise of Isis Isis fighters in Iraq The body of an Isis militant killed during clashes with Iraqi security forces on the outskirts of the city of Samarra Reuters In pictures: The rise of Isis Iraq crisis Iraqi civilians inspect the damage at a market after an air strike by the Iraqi army in central Mosul EPA In pictures: The rise of Isis Iraq crisis Members of the Al-Abbas brigades, who volunteered to protect the Shiite Muslim holy sites in Karbala against Sunni militants fighting the Baghdad government, parade in the streets of the city AP In pictures: The rise of Isis Iraq crisis Shia tribesmen gather in Baghdad to take up arms against Sunni insurgents marching on the capital. Thousands have volunteered to bolster defences AFP/Getty In pictures: The rise of Isis Iraq crisis A van carrying volunteers joining Iraqi security forces against Jihadist militants. Prime Minister Nuri al-Maliki announced the Iraqi government would arm and equip civilians who volunteered to fight AFP/Getty In pictures: The rise of Isis Iraq Fighters of the Isis group parade in a commandeered Iraqi security forces armored vehicle down a main road at the northern city of Mosul In pictures: The rise of Isis Iraq An Islamist fighter, identified as Abu Muthanna al-Yemeni from Britain (R), speaks in this still image taken undated video shot at an unknown location and uploaded to a social media website. Five Islamist fighters identified as Australian and British nationals have called on Muslims to join the wars in Syria and Iraq, in the new video released by the Isis In pictures: The rise of Isis Iraq Al-Qa’ida inspired militants stand with captured Iraqi Army Humvee at a checkpoint belonging to Iraqi Army outside Beiji refinery some 250 kilometers (155 miles) north of Baghdad. The fighting at Beiji comes as Iraq has asked the U.S. for airstrikes targeting the militants from the Isis group. 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For militant groups, the fight over public perception can be even more important than actual combat, turning military losses into propaganda victories and battlefield successes into powerful tools to build support for the cause In pictures: The rise of Isis Iraq An injured fighter (C) from the Isis group after a battle with Iraqi soldiers at an undisclosed location near the border between Syria and Iraq In pictures: The rise of Isis Iraq Fighters from the Isis aiming at advancing Iraqi troops at an undisclosed location near the border between Syria and Iraq In pictures: The rise of Isis Iraq Fighters from the Isis group taking position at an undisclosed location near the border between Syria and Iraq In pictures: The rise of Isis Iraq Fighters from the Isis group inspecting vehicles of the Iraqi army after they were seized at an undisclosed location near the border between Syria and Iraq In pictures: The rise of Isis Iraq One Iraqi captive, a corporal, is reluctant to say the slogan, and has to be shouted at repeatedly before he obeys Sky News In pictures: The rise of Isis Iraq Iraqi captives held by the extremists Sky News In pictures: The rise of Isis Iraq Iraqi captives held by the extremists Sky News In pictures: The rise of Isis Iraq Militants of the Isis group force captured Iraqi security forces members to the transport In pictures: The rise of Isis Iraq Militants of the Isis group transporting dozens of captured Iraqi security forces members to an unknown location in the Salaheddin province ahead of executing them In pictures: The rise of Isis Iraq A major offensive spearheaded by Isis but also involving supporters of executed dictator Saddam Hussein has overrun all of one province and chunks of three others In pictures: The rise of Isis Iraq Militants of the Isis group executing dozens of captured Iraqi security forces members at an unknown location in the Salaheddin province In pictures: The rise of Isis Iraq Isis militants taking position at a Iraqi border post on the Syrian-Iraqi border between the Iraqi Nineveh province and the Syrian town of Al-Hasakah In pictures: The rise of Isis Iraq Isis rebels show their flag after seizing an army post AFP/Getty Images In pictures: The rise of Isis Iraq Isis militants waving an Islamist flag after the seizure of an Iraqi army checkpoint in Salahuddin Getty Images In pictures: The rise of Isis Iraq Demonstrators chant slogans as they carry al-Qa’ida flags in front of the provincial government headquarters in Mosul, 225 miles (360 kilometers) northwest of Baghdad. 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In the house in al-Shadadiya the four girls, all teenagers, were kept in one room with Ms Mueller, who tried to protect them from abuse and violence at the hands of Umm Sayyaf and Isis men, sometimes putting herself in greater danger by doing so, they say. The girls still speak fondly of Ms Mueller today.

Ms Mueller, from Prescott, Arizona, was described as a “mother figure” for them. But she left them regularly, when Abu Bakr al-Baghdadi visited. The girls, who understand Arabic, say they recognised Abu Bakr and other Isis leaders who arrived at the house. Ms Mueller was taken to Abu Bakr’s room elsewhere in the house by Abu Sayyaf. “I saw him often, he came there often,” the Yazidi girl says. “And every time he came, he took Kayla away. She would always come back after.” Every time she returned to the room Ms Mueller told the Yazidi girls what had happened, often crying, telling them that Abu Bakr had sexually abused her – or, in the conservative language the girls use, that “he had married her by force”.

Members of the minority Yazidi sect who were newly released hug each other on the outskirts of Kirkuk April 8, 2015 (Reuters)

A source close to the Mueller family confirmed the girls’ story was correct, to their knowledge. The family have reportedly been left angry by claims that Ms Mueller had converted to Islam or had been a willing captive. Last night Ms Mueller’s parents confirmed to ABC News that government officials have told them their daughter was the victim of repeated sexual assaults by Abu Bakr. “We were told Kayla was tortured, that she was the property of al-Baghdadi. We were told that in June by the government,” Carl and Marsha Mueller said.

The men of the “caliphate” do not usually marry Yazidi women, unless they have converted officially to Islam and have been accepted as Muslims. After the Isis capture of the Mount Sinjar area in August 2014 many thousands of Yazidi were forced into captivity and became sex slaves.

In Abu Sayyaf’s house, Ms Mueller was not the only one to suffer. One of the Yazidi girls had also been given to Abu Bakr, while another one was “married” to Abu Sayyaf, and yet another to Abu Tamim, who was responsible for bringing new Yazidi girls to the house.

When the 14-year-old escaped with her sister in October 2014, she asked Ms Mueller to join her. But Ms Mueller refused, saying her foreign appearance would only put the girls in danger when they would be forced to ask for help from the local Arab community.

About a month after they arrived in the Kurdistan region, in November 2014, the girl and her sister were questioned in detail about their stay with Abu Sayyaf by “some Americans”, the girl says. These people, who the girls believe were intelligence officers, let them go without offering anything in return for their information. US officials last night confirmed that her account had been corroborated with other intelligence.

Smoke rises following an airstrike by a US-led coalition aircraft in Syria (Getty)

The precise circumstances of Ms Mueller’s death remain disputed. Isis claimed she was killed in a Jordanian air strike near Raqqa, the group’s de facto capital in Syria. That has not been confirmed. It has been reported that the bombing raid was targeting Abu Bakr, and Isis is said to have sent Ms Mueller’s family an email with three pictures of her body in a hijab.

Announcing her death in February, President Barack Obama said: “No matter how long it takes, the United States will find and bring to justice the terrorists who are responsible for Kayla’s captivity and death.” Ms Mueller, he said, had “epitomised all that is good in our world”.

In May 2015 an American Delta team targeted the house in al-Shadadiya, and during the operation Abu Sayyaf was killed and his wife captured.

The team found a trove of information on computers and USB-sticks, suggesting that the house played an important role in the command structure of Isis.