On the screen flickered a blurred picture: a tall, grey-bearded man in a white turban talking in Arabic to a number of other similarly attired associates. These fuzzy, broken images were the 'smoking gun' - bin Laden's long-awaited, albeit apparently unwitting, personal confession.

The men in the Afghan Hotel had a more pressing, more local interest too. According to the American authorities who solemnly released the tape last week, it had been found nearby, in a ruined building once used by Arab fighters from bin Laden's al-Qaeda organisation. It had been seized when Mujahideen fighters entered their city a few hours after al-Qaeda's Taliban protectors had left.

That, the viewers knew, was certainly possible. By the morning after the 'liberation' of Jalalabad, fighters loyal to Commander Hazrat Ali had seized safehouses and training complexes linked to al-Qaeda. Ali has cooperated with the Americans and the story that he had passed the tape on to the CIA was very plausible.

But then doubts began to surface. Why had bin Laden broken his tight security to talk? Why had he not used one of his normally favoured media outlets? Was the tape genuine? Was this indeed bin Laden at all?

The tape was certainly damning. It showed bin Laden laughing and boasting about the 11 September attacks as he talks to his interviewer, a Saudi cleric who has travelled through war-racked Afghanistan to see him. Bin Laden, flanked by two key aides, describes how the planes did far more damage to the World Trade Centre than he ever imagined. 'We calculated in advance the number of casualties from the enemy... that the floors that would be hit would be three or four floors,' he says. 'I was the most optimistic of them all.'

Bin Laden also indicates that the men who carried out the plot knew that they were on a 'martyrdom operation' but did not have details of the mission until the last minute.

This weekend, as the debate the tape has provoked continued across the Islamic world, several intelligence sources have suggested to The Observer that the tape, although absolutely genuine, is the result of a sophisticated sting operation run by the CIA through a second intelligence service, possibly Saudi or Pakistani.

'They needed someone whom they could persuade or coerce to get close to bin Laden and someone whom bin Laden would feel secure talking to. If it works, you have got the perfect evidence at the perfect moment,' said one security source. 'It's a masterstroke.'

The focus of suspicion is the Saudi dissident preacher who appears to have taped the interview, conducted according to the timecode on the video on 9 November, in what appears to be a guesthouse in the Afghan city of Kandahar. Though unidentified in the one-hour recording, security sources have told The Observer that the interviewer, who appears to be disabled from the waist down, is Ali Saeed al-Ghamdi, a former assistant professor of theology at a seminary in Mecca. Saudis who watched the tape said the interviewer's accent betrayed roots in the south-west of their country, either the lower Hejaz or Asir province, where most of the 15 Saudi hijackers were from. Bin Laden bows down to greet the cleric, who has not stood up to greet him. Only someone who was incapable of rising would not be on his feet in the presence of such a famous and revered man, Islamic experts said yesterday.

Al-Ghamdi, who is known to Saudi intelligence services, is a marginal figure who tried to make a name for himself through inflammatory anti-Western speeches before being banned from preaching in 1994, one Saudi close to the government said. In the late Nineties he preached in obscure mosques along the highway leading from the port city of Jedda - where bin Laden grew up - to the holy city of Mecca, but his firebrand oratory drew only small audiences.

Senior Saudi government figures and religious scholars tend to dismiss such men as insignificant. 'They are not big-time and they don't have the legitimacy and the religious scholarship that the big guys have,' said Nawaf Obeid, a Saudi security analyst. 'They make a name for themselves with how extreme they are. They aggrandise themselves by claiming they are with bin Laden.'

Security sources stress that, despite his Islamist credentials, al-Ghamdi would still be a potential point of contact for Pakistani, Saudi or Egyptian intelligence.

'He was known because he was suspected of being involved in the gathering of international finance for al-Qaeda. He is a peripheral figure who wants to be something bigger and is frustrated. It's a classic profile. They could have turned him,' one security official for a Gulf intelligence agency contacted in Peshawar said. Experts told The Observer that the tape bears a marked resemblance to secretly filmed evidence used by the FBI against major American Mafia figures in recent years.

And though US security officials said there was 'no confirmation' that the tape was made by an 'intelligence source', a Pentagon official confirmed to The Observer that 'curious circumstances' surrounded al-Ghamdi, who appeared to be aware of the taping.

Whatever its provenance, the video has polarised opinion in the Arab world. 'The vast proportion of people always believed he did it and condemned him for it. They have not altered their view,' said Abdul Wahab Badrakhan, the deputy editor of al-Hayat newspaper. 'Only those with a fanatical mindset would deny what they can now see.'

One such man is Syed, a 38-year-old man who fought alongside bin Laden in Afghanistan during the Eighties. 'There is no way Osama would have done something like this,' he said. 'He was a quiet man with great reverence for human life. The Osama I see happily describing people dying is not the Osama I knew and loved.'

Images of the 44-year-old Saudi-born dissident, who studied civil engineering as a young man, laughing as he talks of how he used his specialist knowledge to calculate how much damage the planes would do, have been difficult for supporters to explain.

General Hamid Gul, a hardline former head of Pakistan's ISI spy agency, suggested that the figure in the video might be a lookalike. Others have queried the translation of the poor-quality Arabic soundtrack and the way that certain key elements - such as the location where the film was made - are inaudible.