Kenya siege: BBC reveals how militants carried out attack
27 September 2013 Last updated at 17:00 BST
The Islamist militants involved in the Kenyan siege rented a shop in the Westgate shopping complex where they were able to stockpile weapons and ammunition, senior security sources have told the BBC.
A BBC investigation into how the terrorists carried out the attack has also uncovered intelligence failures by the authorities.
Karen Allen reports from Nairobi.