Authorities believe the group had planned long in advance, scouting the mall carefully.
"They likely had cased the location for some time and knew very well the best place and time to attack," House Intelligence Committee Chairman Mike Rogers, R-Mich., said in a statement to The Associated Press.
The gunmen, for the most part, are dressed casually. Many are in khakis and long-sleeved shirts. Some have checked scarves around their necks or flung over their heads. Only some are wearing bulletproof vests.
Most carry AK-47 or G3 assault rifles, weapons widely used in the region and easily available on the black market.
But some of the gunmen are draped with belts of large-caliber ammunition, and witnesses hear the fast, frightening, echoing blasts of heavy machine-gun fire.
As they storm through the mall, the music system keeps playing, an undertone to the explosions and screams. The music of Adele and Ne-Yo filters through the carnage.
Millionaires Casino crawl space, 12:57 p.m.:
"Are you okay???"
"Can u message us Mum???" — Text messages Khan received from her 24-year-old daughter while in hiding.
Parking area, third-level rooftop, about 1:30 p.m.:
The young mother watches the gunman shoot. Crowds of people are stumbling, screaming, falling around her.
He is calm.
She is terrified.
Sneha Kothari-Mashru, 28 and a part-time radio DJ, watches through a tangle of her long brown hair, which she has thrown across her face to appear as if she is already among the dead. She has smeared blood onto her arm and her clothes, taking it from the corpse of a teenage boy. She has kicked off her blue high heels.
The gunman doesn't scream, she recalls days later. He rarely speaks. There is no obvious anger in his expression. He seems confident, she says. "He was normal."
About 15 minutes later, Kothari-Mashru watches as the gunman speaks quietly to one family. She can't hear what is said, but the wife is dressed in the billowing robes worn by highly observant Muslim women. Slowly, the family members stand, raise their hands above their heads, and walk away.
Other witnesses described similar scenes. Elijah Kamau, who was at the mall at the time of the midday attack, said he listened as militants told one group of their plans.
"The gunmen told Muslims to stand up and leave. They were safe," he said.
In the email statement, al-Shabab said their fighters "carried out a meticulous vetting process at the mall and have taken every possible precaution to separate the Muslims from the Kuffar (disbelievers) before carrying out their attack."
This is not the rule, however, in the attack.
Dozens of Muslims are shot, and many are killed. Most often, the gunmen fire wildly, spraying bullets into crowds and not bothering to ask about religion.
Some of the bloodiest scenes occur just a few feet from where Kothari-Mashru pretends to be dead.
A Junior Super Chef cooking competition was being held in the parking area and dozens of people — many from Kenya's community of Ismaili Muslims — were at long tables set up beneath car advertisements.
Gunmen had already fired through the crowds at the competition when Kothari-Mashru hides nearby. Afterward, the tables are still arranged in many places, complete with upholstered chairs and red tablecloths. But puddles of blood are everywhere, with corpses one on top of another.
Parking area, third-level rooftop, about 3 p.m.:
Word goes out that someone has found a place to hide.
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