After their initial spree of killings a group of the Islamists who had stormed Westgate dived into the multi-storey Nakumatt supermarket. Security forces pursuing them began to come under steady fire: six of them were shot dead before, outgunned, they pulled out.
The stream of bullets had not come from just Kalashnikov AK-47 rifles the fighters had been carrying, but heavy calibre machine-gun. Later, there were booby-trapped bombs; some police officers also reported the 'whoosh and boom' of grenades being aimed towards them striking walls.
It is highly unlikely that the bulky versions of GPMGs (General Purpose Machine-Guns) IEDs (improvised explosive devices) and RPG (Rocket Propelled Grenade) launchers could have been carried into the shopping mall by the terrorists who had initially gained entry pretending to be tourists even if some of them were, as the authorities claimed, men enveloped in niqabs.
It is almost certain that these had been secreted beforehand, say security sources. It is also likely that quite extensive reconnaissance had been carried out beforehand and that the attackers had detailed knowledge of the vast complex including floor plans. One line of inquiry being pursued by the Kenyan police is that they rented retail premises for their arms, which could also have given them access to the paperwork.
The assault in Nairobi which has left at least 72 dead and 200 injured and missing was an operation plotted in detail and well in advance by al-Shabaab, according to Western officials with knowledge of what is being pieced together.
A key element was the gathering of foreign based jihadists with knowledge of English who are likely to have carried out the early research into the shopping centre, blending in among the expatriates and middle-class Kenyan customers. The sustained media campaign by the Islamists which sprang into life during the attack, with endless tweets and online postings, also suggest at large-scale in planning.
The country's president, Uhuru Kenyatta, said that "five terrorists" have been confirmed killed so far at Westland. The bodies of others, say officials, may be found under the collapsed roof of the complex. But there are signs that others may have fled in the guise of escaping hostages.
Security officials present a 'mini-Mumbai' scenario in which extensive reconnaissance was carried out beforehand. In the assault on the Indian city the key player was Daoud Sayed Gilani, a Pakistani American, also known as David Headley, on behalf of the Lahskar-e-Taiba group based in Pakistan.
A British defence source said: "Mumbai was of course a far more complex op with multiple locations and [Lahskar-e-Taiba] had the backing of the resources of an official agency [the Pakistani secret police ISI] but they used Headley because he could use his American personae without raising undue suspicion. There were supposed to be others like him who were never caught.
"Al-Shabaab would have used their foreign passport holders because they would have found it easier to move about and operate than rural Somalis being brought across the border. This was hardly a spontaneous event, the way these guys deployed, the way they carried out tactical withdrawals, the way they were able to hold out for so long, the fact that they could replenish their ammunition shows a lot of pre-planning."
A British passport holder arrested at Nairobi airport trying to leave the country was named as 35 year old Mohammed Abdilazak who, according to the police, was carrying out the wrong boarding card for his Turkish Airlines flight which had left six hours earlier, He has claimed that he went frequently to Kenya to see his mother and insisted he had no connection with the attack.
Al-Shabbab has its own grandiosely named External Operations Arm and the organisation will be more than happy to take credit for this 'spectacular'.
It has carried out mass murder outside Somalia in the past, 70 people watching the World Cup final in the Ugandan capital Kampala in 2010. But that was done by suicide bombing and it was not thought that the group had the capability to carry out such a sophisticated mission.
This has led to intelligence officials investigating whether it received significant help from al-Qa'ida to which it has pledged allegiance.
But the question remains how such an elaborate assault was carried out under the radar of the Kenyan and Western intelligence agencies in a city that has become the international aid and diplomatic hub for the region, presenting obvious targets for jihadists.
Locally, say Western officials, this was the result of complacency. Following the entry of Kenyan troops into Somalia's civil war in October 2011, a series of bombings had resulted in 32 deaths, but there had been no major incidents.
Kidnapping of foreign holiday-makers and threatening the lucrative tourist industry, led to increased security precautions on coastal resorts. But measures taken in Nairobi to protect civilians, including on the public transport systems, high-rise blocks, churches and shopping centres, soon began to get lax.
There is little indication that Western agencies knew what was coming. Although there was said to be 'chatter' picked up in electronic surveillance that something was planned it was nothing, it is claimed, conclusive.
Sir Jonathan Evans, the then head of MI5, had warned three years ago that young British Muslims were increasingly switching from Pakistan to Somalia to receive training for jihad. Since then a number trying to cross from Kenya into Somalia have been turned back, others have been arrested, including Jermaine Grant who faces charges of possession of explosives intended for bombings in Mombasa, as was another man subsequently charged with murder in the UK. Habib Ghani, a UK citizen from Hounslow in west London, is reported to have been shot dead along with Omar Hammami, an American from Alabama with a Syrian mother, in an al-Shabaab internal feud earlier this month.
The focus, however, is very much on Samantha Lewthwaite, the 29 year old 'White Widow' whose husband, Jermaine Lindsay, was one of the 7/7 bombers wanted in Kenya for alleged terror offences. A number of survivors of the massacre recall a female terrorist in the group issuing orders. The Kenyan government has given contradictory accounts on whether one of the participants was a woman.
Security officials say there is no evidence yet that Lewthwaite was involved. "She has successfully been in hiding and she is a propaganda tool for the Islamists while she remains free. Why should they sacrifice her in a mission where there was always the chance of getting killed or captured?" Asked one. "Also, as far as we know, she has no combat experience, so hardly in position to give tactical orders."
US officials stated that they were "aggressively investigating" the attack and claims of involvement by a large number of American citizens. But they acknowledged that despite Congressional hearings into mosques recruiting for jihad in Somalia and stepping up of scrutiny there had been no foreknowledge of what unfolded.