 Between ten and twenty pounds of high explosive had been placed in a red taxi that had been purchased on the morning of 3 March in Edmonton, north London, and abandoned yards from the main front door of BBC Television Centre at 11 pm.Police officers were attempting to carry out a controlled explosion on the bomb with a bomb-disposal robot when it went off.Staff had already been evacuated after police received a coded warning that had been given to a London hospital and charity one hour before the explosion.