Abu Faraj al-Libbi, arrested in Pakistan this week, is a Libyan described by Pakistani officials as the key al-Qaeda operative in the country.
Libbi, top right, on a Pakistani government "most-wanted" advert
But until a year ago, he was a relatively unknown figure in the hierarchy of alleged militants on the run since 11 September.
Libbi's name was first made public in Pakistan last year when it was included in the poster of six most-wanted militants issued by the government.
Along with Amjad Farooqi - a man desperately wanted by Pakistani authorities for his alleged involvement in sectarian militancy inside the country - Libbi was advertised as a "terrorist" with 20m rupees ($340,000) offered for information leading to his arrest.
Informed investigators working for the Pakistani military say Libbi, who suffers from the skin disorder, luecoderma, and used the alias Dr Tawfiq, had not initially been considered a major player in al-Qaeda by Pakistani authorities.
It was only through the interrogation of a number of suspects - arrested between January and August last year - that the Pakistani authorities started taking note of his presence in the hierarchy, investigators say.
One security official said: "Every time we interrogate a militant linked to al-Qaeda, al-Libbi's name pops up."
According to Pakistani investigators, the first breakthrough came with the arrest of Naeem Noor Khan in Lahore in July last year.
Security officials say that Khan's interrogation revealed the extent of al-Qaeda operations in which Libbi and Farooqi were involved.
The two were learned to be actively communicating with al-Qaeda operatives outside Pakistan and in particular in the UK.
The UK media ran several stories on Libbi's alleged plans to "carry out pre-election attacks in the US" for which he was said to be in touch with various UK-based agents through coded messages managed by Khan.
Libbi's exact standing in al-Qaeda remains unclear, but Pakistani and Western intelligence officials generally believe he took over as third in command of the organisation when his mentor, alleged 9/11 mastermind Khalid Sheikh Mohammed, was captured in March 2003.
Farooqi was killed in a shootout with police in Nawabshah in central Sindh province in September 2004.
Musharraf survived two attempts on his life in Rawalpindi in 2003
Security officials say Libbi has since been the main contact between al-Qaeda leader Osama Bin Laden and Islamic militants operating inside Pakistan.
According to Pakistani security officials, the arrest last December of six militants from Karachi provided the authorities with some "critical intelligence" about Libbi's whereabouts.
Libbi's arrest was preceded by a spate of speculation regarding "a major operation" in Pakistan's tribal belt of Waziristan - first hinted at by the head of US forces in Afghanistan, Lt Gen David Barno.
But irrespective of whether Libbi was specifically the focus, his arrest is still being described by Pakistani and Western officials as hugely significant.
President Pervez Musharraf has directly blamed him for financing and supervising two attempts on his life in December 2003.
He is also regarded as the prime suspect in a number of bombing incidents in Pakistan, including last year's attempt to kill Prime Minister Shaukat Aziz.
Libbi's arrest is also a significant indicator of the success of intelligence sharing between Pakistan and the US.
Islamists in Pakistan have repeatedly accused the Musharraf government of letting US agents operate on Pakistan soil. But Pakistan has consistently held that their cooperation with the Americans is limited to the sharing of information.
Pakistani security officials are now citing Libbi's arrest as a classic example of the success of this cooperation, whereby the US supplies Pakistan with the required intelligence and Pakistani forces carry out the actual operations.