The Hunt for bin Laden
From Afghanistan to Abbottabad.
The death of Osama bin Laden in May this year brought the long search for the world's most wanted terrorist to a final end.
It had begun well before 9/11, but that event focused both minds and resources on the mission.
A special team was despatched to Afghanistan where bin Laden was under the protection of the Taliban.
Their leader was given special instructions to 'bring back bin Laden's head in a cardboard box'.
Today Cofer Black, head of the CIA's Counter Terrorism Unit and the man who gave the order, explains why:
"If you have engaged the target, you have to have a compelling proof that you've been successful.
When you go up to your commander in chief and say 'We got him', what's the next question? 'Are you sure?'.
The team never did manage to find bin Laden.
Their best opportunity was in the Tora Bora mountains where the al-Qaeda leader took refuge in the caves he had known from his days fighting Soviets.
The team was accompanied by US Special Forces but asked for extra troops as their Afghan allies were proving reluctant to fight.
Their request was denied, a decision defended in the programme by General Richard Myers, former Chairman of the Joint Chiefs of Staff.
Osama bin Laden is thought to have escaped across the border to Pakistan where American troops were unable to follow.
Various attempts were made to find out where he was over the following years but to no avail.
A hotel owner describes how two mysterious Americans rolled up with night vision goggles in Northern Pakistan and Gary Faulkner, a former Colorado construction worker, talks about his own mission to find bin Laden disguised in local dress and armed with a sword.
Bin Laden, meanwhile, continued to taunt the world by releasing the occasional video, proving that he was still alive.
In the end, one of bin Laden's couriers was to provide the breakthrough which led to the raid on the compound in Abbottabad earlier this year.
Intelligence officials talk about the use of harsh interrogation techniques (or torture in some people's eyes) which may have played a part.
Senior members of the current administration describe the lead up to the raid in Abbottabad which, had it failed, could have cost Barack Obama his presidency.
As it is, there are still those who think the hunt could have ended much sooner, including Richard Clarke, Counter Terrorism adviser under both Bill Clinton and George Bush:
"I hate to use a simplistic analogy but if you order a pizza delivery that shows up fifteen years later it might be a really good pizza but it's still fifteen years late."
Presented by the BBC's security correspondent Gordon Corera.
Producer: Mark Savage.
Gordon Corera on the search for the world's most wanted terrorist.