Calm amid the killing: Kenya CCTV shows Westgate gunmen

The footage shows four young men ambling around with AK-47 rifles in hand.

NAIROBI: With chilling nonchalance, gunmen who massacred at least 67 people wander through Kenya's Westgate mall, seen for the first time in security camera footage providing possible vital evidence to investigators.

Calmly searching corners of the mall - possibly hunting for more victims to kill after executing scores in the main hall of the upmarket complex - the closed-circuit television footage shows four young men ambling around with AK-47 rifles in hand.

Witnesses in the mall described how the fighters stormed the complex midday Saturday when it was crowded with shoppers, firing from the hip and hurling grenades at shoppers and staff.

But hours after the attack started, the men are seen in the bread section of the supermarket, the bulk of their victims most likely already dead.

They wear backpacks, perhaps stuffed with the ammunition that they would use to keep Kenya's army - backed by foreign special forces - at bay for four days.

Two weeks since Somalia's al Qaeda linked Shebab insurgents attacked Nairobi's upmarket Westgate mall, the CCTV footage could help answer questions about the attack.

One thing is clear: the men were organised, apparently unafraid, and utterly ruthless.

During the attack, Kenyan officials said that between 10 - 15 gunmen were involved, and later claimed to have killed five, although it is not clear where those bodies are now.

But the footage released so far, which covers only one part of the mall and which was taken around five hours into the 80-hour siege, shows only four men.

In one shot, one of the attackers looks at supermarket checkout tills, knocking over computer screens.

At the same time, he is seen chatting on a mobile phone, suggesting either coordination with attackers inside the mall or even to commanders outside or in Somalia.

The Shebab issued a running commentary throughout the attack via various Twitter accounts, claiming to be in contact with the fighters inside the mall.

In addition, no female attackers are seen, in line with Shebab claims all the insurgents were men, and suggesting reports that wanted British 'White Widow' Samantha Lewthwaite was involved were false.

It could provide key clues for detectives, not least because the supermarket area - where the footage was shot - was later engulfed in a fierce blaze.

Kenyan and foreign investigators are continuing to comb the carcass of the mall for clues to the perpetrators and their operating methods.

Part of the rooftop parking of the mall collapsed on top of the supermarket, leaving the key site where the insurgents are understood to have made their final stand - along with possible hostages - buried under tonnes of concrete slabs and rubble.

Thirty-nine people are still listed as missing by the Red Cross.

It is not clear what happened to those seen in the footage: whether they died in a shootout as Kenya claims, or in the fire that gutted part of the mall, or indeed, if they managed to escape.

The Shebab have warned they will step up militant attacks against Kenya, after Nairobi refused to pull its troops out of Somalia, warning last week in the latest in string of threats that "rivers of blood will flow in Nairobi."

Kenya invaded southern Somalia to attack Shebab bases two years ago, and later joined the 17,700-strong African Union force deployed in the country.

Meanwhile foreign special forces staged a nighttime sea and air attack on a key Shebab base in southern Somalia, apparently targeting a senior commander. The Shebab said they fought off the assault.