(CNN) -- As they strolled through Westgate Mall, guns strapped to their torsos, the attackers chatted on their cell phones while they sprayed bullets at terrified shoppers.
Ruthless and nonchalant, they randomly gunned down shoppers at the upscale mall in the Kenyan capital.
At one point, they took turns to pray, removing shoes to perform the ritual washing in a room stacked with boxes. They bowed down in Islamic prayer, taking a break from incessant gunfire.
Chilling footage from mall
Closed-circuit television footage obtained by CNN provides the chilling new details of what happened during the attack last month.
In part, it shows two people with guns casually walking and shooting their way through a supermarket in the mall. During the hours of video, they are seen talking on the phone, praying and shooting toward anything that moved.
A man whimpering in a pool of blood on the floor crawled to get away. A gunman returned and shot him, again.
In between the gunfire, the attackers scanned ceilings for surveillance cameras. Nearby, shoppers hid behind cash registers while some ran for their lives. Others, too terrified to move, cowered on bloody floors.
Parking lot mayhem
The footage was taken on September 21, day one of the four-day siege. Though the video has no sound, the terror on the faces of the victims is loud and clear.
In addition to the two attackers in the supermarket, the footage shows two more making their way through the parking lot. They opened fire. Shoppers slithered under cars. Others tumbled to the ground, felled by bullets.
The attack left at least 67 people dead, many more injured and dozens unaccounted for.
Details match up
Details from the footage appear to corroborate a running commentary by the Al-Shabaab, which claimed responsibility for the attacks.
In its Twitter account during the siege, the terror group said the attacks were conducted by men. It denied reports that terror suspect Briton Samantha Lewthwaite was involved.
Al-Shabaab said it remained in contact with the attackers as they battled Kenyan forces during the hostage crisis.
Hostage takers join forces
The attackers in the parking lot later joined the others in the mall. The four wandered, rounding up hostages.
A woman with two children and a third in a shopping cart walked by a line of cash registers. A bloody teenage girl followed them; a gunman pointed the way. The woman, the children and girl were all eventually released.
The footage focuses on the corner of the mall closest to the supermarket, which occupies the anchor position in the four-story mall. The stories repeated themselves in various places at the mall, which has more than 80 stores.
There are many mysteries about the mall attack.
With dozens still unaccounted for, how many people did the attackers kill? How many terrorists did the Kenyan forces kill? How many civilians and terrorists are buried beneath the concrete chunks of the collapsed section of the mall?
It's unclear whether the four terrorists in the video were arrested or gunned down by security forces at the scene. Even the total number of assailants is unknown.
At the time of the attack, Kenyan officials said that up to 15 gunmen were involved.
On day four, as the operation wound down, President Uhuru Kenyatta said five terrorists were killed and 11 suspects were in custody.
But since then, the numbers have fluctuated, with different officials citing various numbers between four and six.
This month, military spokesman Maj. Emmanuel Chirchir said four gunmen died when part of the mall collapsed. It's unclear whether he was referring to those in the video. Their charred bodies, he said, are undergoing DNA tests.
Chirchir also released the names of the dead militants, including an American of Somali origin, but analysts say those names may be pseudonyms.
Officials have said the answers may be buried beneath the rubble of the collapsed section of the mall.
But, as they scour through the mangled heap, there are more questions than answers.