A TIP-OFF about a courier thought to be particularly close to Osama bin Laden was the crucial piece of evidence that led to the death of the al Qaeda leader after a decade-long manhunt.

Although US authorities became aware of a man who was bin Laden’s most trusted courier shortly after the September 11 terrorist attacks, the true identity of Kuwaiti-born Sheikh Abu Ahmed, who was killed in the same raid that left bin Laden dead, remained unknown for several years.

The potential significance of the courier and his links to the world’s most wanted man was first mooted by CIA detainees in late 2001 but, at first, intelligence officials knew him by his nom de guerre, Abu Ahmed al-Kuwaiti.

One of the key sources for information was Khalid Sheikh Mohammed, the al Qaeda operative said to have masterminded the attacks on the World Trade Centre and the Pentagon.

In Guantanamo Bay, Mohammed was waterboarded 183 times without identifying Abu Ahmed but later, under more conventional interrogation techniques, he eventually confirmed knowing him.

In 2004, al Qaeda operative Hassan Ghul was captured in Iraq and, although he was not taken to Guantanamo, while in US custody he went on to confirm to officials that Abu Ahmed was a crucial figure within the terrorist organisation. He indicated that the courier was close to al Qaeda’s third in command, Faraj al-Libi.

This information gave the US the detail they needed and yesterday one official described Ghul as the “linchpin” in finding and killing bin Laden.

The following year, Faraj al-Libi himself was captured and he appeared to confirm Abu Ahmed’s close connection to bin Laden.

Al-Libi admitted that he was given his own position as number three in al Qaeda, replacing Mohammed, through a courier but denied knowing Abu Ahmed personally.

By 2007, intelligence officers had managed to discover Abu Ahmed’s real name and soon the net was closing in on the courier.

US Government files said to have been leaked to WikiLeaks apparently show that he was an IT expert who helped to give computer training to members of the team involved in the plot to blow up four planes over US soil in September 2001.

One document reportedly refers to information obtained from Muhammad Mani al-Qahtani, who is described as the 20th hijacker of the September 11 attacks and who had attempted to enter US soil just a month before the atrocity.

Dated October 30, 2008, the document states: “Detainee received computer training from al Qaeda member Abu Ahmed al-Kuwaiti in preparation for his mission to the US.”

Separate information included in a file on Khalid Sheikh Mohammed states that he ordered Abu Ahmed to train al-Qahtani in sending emails because “email was safer than talking on the phone”.

It states: “Abu Ahmed al-Kuwaiti was a senior al Qaeda facilitator and subordinate of (Mohammed). Al-Kuwaiti worked in the al Qaeda media house operated by (Mohammed) in Kandahar and served as a courier.”

According to the files passed on by WikiLeaks, Abu Ahmed may have been with bin Laden as early as late 2001, when al Qaeda was said to have been based in the Tora Bora caves.

They say: “Al-Kuwaiti was seen in Tora Bora and it is possible al-Kuwaiti was one of the individuals reported accompanying (bin Laden) in Tora Bora prior to (bin Laden’s) disappearance.”

US officials have been at pains to stress that the first key intelligence reports identifying Abu Ahmed were received the year after interrogation techniques such as waterboarding were suspended.

But the possibility that US spies located bin Laden with help from detainees who had been subjected to “enhanced interrogation” procedures seems certain to reopen the debate over practices that many have equated with torture, security experts said.

Paul Wolfowitz, George Bush’s deputy defence secretary, said the successful operation against bin Laden showed the value of the Bush administration’s interrogation policies.

He said: “This would not have been possible if we were releasing terrorists willy-nilly and not keeping them for the information they had, some of which often may not look that important, like the pseudonym of a driver, until it turns out that he’s really a critical person.”

It took until August last year for US military to track Abu Ahmed to the compound in Abbottabad in Pakistan and in the end, it was a simple phone call that revealed bin Laden’s hiding place. Abu Ahmed had reportedly phoned an al Qaeda operative being tracked by the CIA and this led to a shift in focus to Abbottabad.

Tom Shields: Page 16