Three weeks after the first London bombings, British and American security sources are giving markedly different versions of how much was known about the bombers before the attacks and who masterminded them.

According to US intelligence sources, a man now being held in Zambia is Haroon Rashid Aswat, a Briton of Indian origin who has links to a convicted Al-Qaeda terrorist. They believe he assisted or masterminded the London attacks.

But British investigators, examining whether telephone calls were made between the London bombers and Aswat before the attacks of 7/7, caution that the calls may have been made to a phone linked to Aswat, rather than the man himself.

Some of the mobile phones used by the 7/7 bombers have been recovered from the scenes of the explosions. Even though they are badly damaged, forensic telecommunications experts have had some success in recovering vital data relating to outgoing calls, text messages and voice mail.

Those details are allowing investigators to draw up a network of “concentric circles” around the four dead men, an exercise that has already led them to identify some of those who may have helped the bombers.

This weekend it appears that several calls from Aswat’s mobile telephone were made to the bombers in the days before the attacks. It is likely that the American National Security Agency — which has a powerful eavesdropping network — was monitoring the calls. If contacts between the bombers and Aswat are proved, it could be a painful blow for British security officials.

In the weeks before the attacks Aswat, according to American officials, was under surveillance in South Africa and US authorities wanted to arrest him for questioning.

The South Africans are believed to have relayed the request to British authorities who were reluctant to agree to him being seized because of his status as a British citizen. The US, it is claimed, wanted to take control of Aswat using a process known as “extraordinary rendition”, which would bypass the normal extradition process and may have resulted in him being flown to Guantanamo Bay in Cuba or a country that allows torture.

However, questions are also being asked about whether the British did not wish to have Aswat arrested because he was seen as a useful source of information. To some, British intelligence is too willing to let terrorist suspects run in the hope of gathering useful leads and other information.

In the weeks before the London attacks a man said to be Aswat may have entered the UK, though British security officials think this may be a case of mistaken identity.

What seems clearer is that he either slipped his surveillance or was allowed to move on from South Africa. He was seized in Zambia on July 21, according to the Foreign Office, the day the second wave of would-be suicide bombers struck. On Friday, British officials had yet to be granted access to him.

As a potential mastermind of the London attacks, Aswat has connections and a past that are almost too neat a fit. Now 31, he was brought up in Dewsbury, near Leeds, where Mohammad Sidique Khan, one of the London bombers, lived. He left the area 10 years ago and is believed to have travelled to training camps in Pakistan and Afghanistan. He is said to have told investigators in Zambia that he was once a bodyguard for Osama Bin Laden.

When Aswat returned to Britain he attended the Finsbury Park mosque in north London, which was a hotbed of radicalism in the late 1990s and early 2000s.

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