“It was 99 percent an inside job,” said a senior Iraqi police official on condition of anonymity. “We found TNT inside the jail. Some prisoners managed to convert soap and water bottles into IEDs (improvised explosive devices).”

Investigators also found laptops and mobile phones in the prison cells, indicating the inmates had been in direct contact with their comrades all along. Memory cards had been removed from surveillance cameras by the guards.

“Unfortunately guards who work in these places are facing serious temptations, mostly money and women,” the officer said.

Militants had warned residents in the surrounding area two days in advance of their plans to storm Abu Ghraib. Several officials said the security services had received intelligence about the operation, but failed to act on it.

Once the prisoners scaled the walls, bridging the razor wire with their mattresses, cars were waiting to whisk away the highest-profile al Qaeda members, probably to neighbouring Syria, where they will be welcomed as heroes by fellow militants fighting to overthrow President Bashar al-Assad.