The attack came in the midst of the Northern Iraq offensive, when the Islamic State was seizing territory across the country and the Iraqi government was fighting back with the help of Shia militias. Two bombs had detonated at the house of Abdul Samad Salar Al-Zarkoshi, a local Shia militia leader, killing three of his men. What followed was retribution directed not at combatants but at civilians kneeling in prayer. Masked gunmen stormed the mosque and opened fire with automatic weapons. Witnesses described eight men, some in plain clothes, others wearing police uniforms. One attacker wore a green headband bearing the name of the militia Asa'ib Ahl al-Haq. The imam was the first to fall. Women who rushed to the mosque to find their relatives were shot as well.

The survivors who dragged wounded worshippers into the mosque's garden discovered the attack was not over. Between twenty and thirty armed men advanced on the building, firing as they came, finishing off the injured. When Iraqi security forces finally arrived, supported by Shia militia units, they triggered bombs the attackers had planted to cover their escape. Four militiamen died in the blasts and thirteen were wounded. Lawmaker Nahida al-Dayani, a Sunni originally from the village, described the scene in plain terms: most mosques had no security, some of the victims were from a single family, and some of the dead were women who had simply come to learn the fate of loved ones.

The massacre sent shockwaves through Iraq's political system. Sunni politicians Saleh al-Mutlaq and the newly appointed speaker of parliament, Salim al-Jabouri, suspended their participation in talks to form a new government with the main Shia political alliance. The National Iraqi Alliance condemned the attack but criticized the suspension, arguing it served the perpetrators rather than the victims. Other Shia militias, including ironically Asa'ib Ahl al-Haq itself, called the attack barbaric. UN Secretary-General Ban Ki-moon condemned it in the strongest terms, declaring attacks on places of worship completely unacceptable under international law. Four days later, three brothers from the al-Zarkoshi tribe were arrested near Baqubah. Their brother, the militia leader, was reported to have fled to Iran.

On November 2, 2014, Human Rights Watch published its findings. The investigation confirmed what witnesses had described: Asa'ib Ahl al-Haq, a Shia militia operating alongside Iraqi government forces, had carried out the massacre. All seventy-three dead belonged to the Bani Wais tribe. The report detailed accounts from survivors who had watched armed men systematically execute the wounded in the garden of a place of worship. The Imam Wais village sits in flat, agricultural land along the Diyala river basin, a landscape that offers no cover and no escape. For the people of Bani Wais, the mosque that bore the name of one of the Prophet Muhammad's earliest companions had been the one place they believed was safe. That belief died on a Friday afternoon in a province where the distance between neighbor and enemy had collapsed to nothing.