They arrived in five SUVs, took positions across from the mosque's door and windows, and just as the imam was about to deliver his sermon from atop the pulpit, they opened fire and tossed grenades at the estimated 500 worshippers inside.
When the violence finally stopped, more than 300 people, including 27 children, had been killed and 128 injured.
As the gunfire rang out and the blasts shook the mosque, worshippers screamed and cried out in pain.
A stampede broke out in the rush towards a door leading to the washrooms. Others tried desperately to force their way out of the windows.
Those who survived spoke of children screaming as they saw parents and older brothers mowed down by gunfire or shredded by the blasts. Some families lost all or most male members in the massacre.
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So composed were the militants that they methodically checked their victims for any sign of life after the initial round of blazing gunfire. Those still moving or breathing received a bullet to the head or the chest, the witnesses said.
When the ambulances arrived they shot at them, repelling them as got back into their vehicles and fled.
Friday's assault was Egypt's deadliest attack by Islamic extremists in the country's modern history.
A statement by the country's chief prosecutor, Nabil Sadeq, said the attackers, some masked, numbered between 25 and 30. Clad in military-style camouflage pants and black T-shirts, one of them carried a black banner with the declaration of the Muslim faith - there is no God but Allah and Muhammad is his prophet. The banner matched those carried by IS.
They also torched seven cars parked outside the mosque that belonged to worshippers, the statement added.
"We knew that the mosque was under attack by (militants)," said witness Ebid Salem Mansour recalling the intense gunfire. Mansour, a 38-year-old worker in a nearby salt factory, suffered two gunshot wounds to his legs on Friday.
"Everyone lay down on the floor and kept their heads down. If you raised your head you get shot," he said. "The shooting was random and hysterical at the beginning and then became more deliberate. Whoever they weren't sure was dead or still breathing was shot dead."
The militants were shouting Allahu Akbar, or God is great, as they fired at the worshippers and the children were screaming, Mansour added.
"I knew I was injured but I was in a situation that was much scarier than being wounded. I was only seconds away from a certain death," he said.
President Abdel-Fattah el-Sissi vowed that the attack "will not go unpunished". On Saturday, he ordered that a mausoleum be built in memory of the victims of Friday's attack.
