"The co-operation was why there were no troops in Abottabad," writes Dr Hillhouse. "It had always seemed very far-fetched to me that a helicopter could crash and later be destroyed in an area with such high military concentration without the Pakistanis noticing." In the immediate aftermath of the raid, some residents of Abbottabad, where bin Laden had lived for five years, said they had received mysterious visits a night earlier warning them to stay inside with their lights off.