September 2013 Last updated at 12:00 ET
Militants 'hired Kenyan mall shop'
The militants who led the attack on a Kenyan mall hired a shop there in the weeks leading up to the siege, senior security sources have told the BBC.
This gave them access to service lifts at Westgate enabling them to stockpile weapons and ammunition.
Having pre-positioned weapons they were able to re-arm quickly and repel the security forces.
Sixty-seven people are known to have died in the four-day siege. Kenya's Red Cross says 61 others are still missing.
Forensic experts are still combing the complex, looking for bodies and clues.
"Terrorism is an exploitation of openings" was the way Ndung'u Gethenji, chair of Kenya's parliamentary defence committee, described to me the attack in Nairobi. And the gunmen at the Westgate siege exploited those openings to the full to mount a "spectacular" that has wounded Kenya - and left its people shaken.
That extremist gunmen could secure a base within the mall in the weeks leading up to the attack and pre-position weapons is in itself astonishing. But for many Kenyans, audacious as it is, it will come as little surprise when bribery remains the currency of everyday life.
A few "bob" - (Kenya shillings) to "look the other way" is not unusual here, despite the best efforts of many brave Kenyans to rein the problem in. Porous borders and a ready supply of weapons have long fuelled the threat of violence in Kenya - an AK-47 costs just $450 (£280) today if you know the right people.
Many Kenyans now hope the legacy of Westgate will be tougher action to tackle dodgy deals. The human cost of not doing so has already been laid bare.
The Somali Islamist group al-Shabab, which is part of al-Qaeda, says it was behind the attack and the following siege at the upmarket mall in the Kenyan capital, Nairobi.
Kenya is in its third day of official mourning for both the civilian and military victims of the attack.Fake IDs?
The BBC investigation has revealed how the Westgate gunmen were able to plan and carry out the siege, and how security breaches allegedly fuelled by corruption made it an attack waiting to happen.
To rent a shop, the militants would have needed fake IDs supplied by corrupt government officials.
The BBC has also confirmed more details about how they executed their attack.
Two vehicles dropped the Islamist extremists off outside before they forced their way into the mall, sources say.
They are also believed to have set up a base using a ventilation shaft as a hiding place, on the first floor.
Security sources have also confirmed a change of tack by the militants late on Saturday.
They rolled out heavy calibre machine guns, exploiting the moment when control of the security operation switched from the police to the military.
There are reports that this switchover was fraught with confusion.
New photos revealing some of the damage at the Westgate shopping centre have been released.
They show how parts of the rooftop car park have collapsed down onto what is believed to be the supermarket area.
The manager of a jewellery shop allowed back into Westgate showed the BBC photos she took, showing that the store had been looted.
President Uhuru Kenyatta declared three days of official mourning this week. The funeral of his nephew and his nephew's fiancee took place in Nairobi on Friday.