Only a few months after 9/11, American troops located Osama Bin Laden in the Tora Bora mountains of Afghanistan - so how was he able to evade them?

As members of the British Special Boat Service (SBS) team listened in to conversations on a captured short wave radio, they heard a voice they believed to be their target.

Two of the team spotted a tall figure in a camouflage jacket moving with a 50-man protective detail, who went into a cave through a hidden entrance.

Only a few months after the 11 September 2001 attacks, Osama Bin Laden seemed to be cornered in the mountains of Afghanistan, close to the Pakistani border.

Tora Bora promised to be his final stand. So how did he escape?

The SBS soldiers had joined an American-led team alongside CIA and US Special Forces who had followed Bin Laden from Jalalabad into the White Mountains and finally to Tora Bora, a remote complex of caves.

Once the team approached the foot of the mountains, they took over a schoolhouse as a base.

Four men headed into the mountains, accompanied by 10 Afghans. It was the most rugged terrain many had ever experienced.

Image caption The multi-storied cave complex at Tora Bora was widely thought to be Bin Laden's headquarters

When they reached an outcrop and saw a large group of up to about 900 al-Qaeda figures, the battle for Tora Bora began.

The commander back at base, Gary Berntsen, issued orders to open fire. He only told headquarters after the fact.

The team called in air strikes over the next 56 hours.

"We threw everything at him. I didn't even know we had that many B52 or B1s," one of the Special Forces soldiers who was on the ground told the BBC, speaking on condition of anonymity.

"Everybody was trying to get into the fight because… he was there. I can tell you that we dropped so much munitions on this place that we actually changed the landscape. The map is different."

'Political calculations'

More soldiers joined the team, but it never numbered more than 100. Three sections made a push up the mountain.

Further conversations overheard on captured al-Qaeda radios indicated that Bin Laden was still alive. The tone of his intercepted communications changed.

"It became more of desperation that doom is coming," recalls one American soldier. The Americans believed he was within a mile and a quarter (2 km) of one of the teams.

THE BRITISH CONNECTION As they moved across Afghanistan pursuing Bin Laden and al-Qaeda, British teams discovered plans for a follow-up attack to 9/11, according to CIA officer Gary Berntsen: "They were the ones that debriefed the individual in question. We had had access to that prisoner. The British did it. "And the British - a very capable British officer - was the one that was able to gain the confidence of an al-Qaeda member who then told him that the follow-on attacks to 9/11 are going to be 20 tonnes of explosives in Singapore, they're going to destroy the US, British and Israeli embassies there." Following the interrogation, Singaporean authorities conducted their first raid on 9 December 2001 and went on to find surveillance video and bomb-making instructions. They said a major plot had been foiled.

Members of that team wanted to push forward but were told to wait since they lacked numbers.

"Every now and again, we'd talk about mutiny and just moving up," a member recalls.

The strategy, as with the whole Afghan campaign, was to limit the number of American boots on the ground.

Instead Afghan fighters would operate under the direction of the small CIA/Special Forces teams, supported by air power.

But at Tora Bora, the Afghan mujahideen proved unreliable allies. They refused to fight at night leaving al-Qaeda to reoccupy ground that had been painfully won.

At one point they agreed a ceasefire which may even have secretly assisted Bin Laden. "I don't think they were properly trained," recalls the anonymous Special Forces soldier. "And I don't think their heart was in it."

Berntsen asked for 800 US Rangers to be placed between Bin Laden and the border, or to enter the mountains from the Pakistani side. His request was denied.

"Once you have to ask Washington for assistance, then all sorts of political calculations enter in and… unfortunately that's what occurred," Berntsen told the BBC.

'Military incompetence'

The commander of the CIA Afghan operation, Hank Crumpton, spoke to the top military commander who said it would take weeks to get troops in.

"Tora Bora was just a case of military incompetence," argues Richard Clarke, at the time, a White House counter-terrorism adviser.

"They had plenty of time, they had the people, they had the information - this was not a matter of miscommunication. This was a matter of general officers deciding not to do it because they didn't think it was their mission."

BIN LADEN: THE LOST YEARS Image caption Mr Faulkner was given a rapturous reception on returning home After Bin Laden's escape from Tora Bora came the lost years, with many rumours but few strong leads. The $25m bounty attracted the odd amateur, among them Gary Faulkner - a Colorado construction worker and former convict who was eventually arrested in Pakistan in local dress carrying a sword. "Look at all the military guys. They're dressed up in all these special op outfits and they carry all this ridiculous gear and all this junk that's fine if you've got a helicopter to drop you," he told the BBC. He says his time in prison gave him a unique skill set: "I got a different type of training because I'm into someone else's backyard whether it's another town, another state - without the special ops and the computers and the satellites. "The objective was not to kill or destroy or blow stuff up. The objective was to go in there quietly as a thief and boost him, and take him back down." Mr Faulkner returned to the US after his arrest in June 2010, but has vowed to go back. US 'Bin Laden hunter' back home

Many are angry that the US did not use marines based in Kandahar, not far away.

Cofer Black, the CIA's director of counter-terrorism at the time, blames the decision on a reluctance to risk troops.

"When you compare that to a war that went on for 10 years with American and British troops fighting, in hindsight you can say… it would have been well worth it," he says.

"If I'd had any wish, it would have been I'd got up off my desk and made a call and said, 'I want to see the president.'"

General Tommy Franks, then in charge of Central Command (CentCom), made the decision not to send in reinforcements. He declined to comment. Other colleagues reject the idea it was a mistake.

"You could have waited longer and put troops in. In that same amount of time, Bin Laden could have left," argues General Richard Myers, then the chairman of the Joint Chiefs of Staff.

"So there was a sense of urgency. And the decision was left to General Franks, the CentCom commander, and we backed him. We thought, 'Yep, that sounds good'. I'd do the same thing again."

Berntsen remains angry. "The US military spends a trillion dollars a year on defence. Why is it that they could not get forces into that area? It's a ridiculous statement that they couldn't get people there.

"We had the marines actually down in Kandahar. They could have been lifted up there. If there is a will there is a way."

A Delta Force officer asked for mines to be laid at the back of the mountains to close off a potential escape route to Pakistan. This was also denied.

"We don't use mines that way," argues Myers, a view which is disputed by some Special Forces individuals involved. Some believe the decision was the result of a commitment made after pressure from the British.

Find out more The Hunt for Bin Laden is on BBC Radio 4 on Friday 15 July and Friday 22 July at 1100 BST. Listen to both episodes via the BBC iPlayer Dead or alive? US indecision on Bin Laden

As the battle ended, the anonymous Special Forces soldier trudged up to the last known location of Bin Laden. It was cold with high winds and the rain was coming down.

The air assault had pulverised the landscape to the point that he remembers wading knee-deep through a strange mixture of sand and dirt created from the bombed rock.

"And the mujahideen come up and they say 'OK. Osama's gone to Pakistan. We're going home.'"

Bin Laden had escaped to Pakistan - his exact route remains disputed - and there he would remain, out of sight, for close to another ten years until another joint CIA/Special Forces mission finished the task that had been set at Tora Bora.

The Hunt for Bin Laden is on BBC Radio 4 on Friday 15 July and Friday 22 July at 1100 BST. Or listen again to both episodes via the BBC iPlayer.