They really want to believe that he is dead

Last week's attempt on the life of the Afghan president, Hamid Karzai, and a car bombing in Kabul the same day which killed 30 people, have been taken as evidence that al-Qa'ida is very much alive and operational. Whether the same can be said of its leader, Osama bin Laden, is another matter entirely.

The US, which led a coalition into Afghanistan to smash al-Qa'ida and its Taliban allies, does not know whether Mr bin Laden is alive or dead, judging by the recent statements of its most senior officials. In July, Vice President Dick Cheney quoted George Bush as saying, "If he's alive, we'll get him. If he's not alive – we got him." Last month Donald Rumsfeld, the hawkish Secretary of Defense, said: "He's either alive and in Afghanistan or somewhere else, or he's dead."

In two Afghan dispatches less than a week apart, The New York Times reported first that he was probably alive, then that he was probably dead. American commanders "appear to have concluded" he was moving between mountain hideouts on the border between Afghanistan and Pakistan, it said on 28 August, only to quote "some senior officers" in the special forces on 3 September as deciding that he was probably killed by American bombing in Tora Bora in December.

Certainly nothing has been heard from the terrorist mastermind since the beginning of the year. One or two videos have surfaced, but they appear to have been filmed some time ago. "My gut feeling is he's dead," one intelligence analyst told The Independent on Sunday last week, but Magnus Ransthorp, a terrorism expert from St Andrews University, said: "I think he is somewhere on the Afghan-Pakistan border. A number of reliable Arab sources are telling me that."

The last undisputed sighting was in early November, when Mr bin Laden was interviewed by Hamid Mir, a Pakistani journalist who had written a biography of him. After going to Kabul at the height of the coalition bombing campaign, Mir said he was rolled in a blanket, put in the back of a vehicle and taken on a five-hour journey to meet the fugitive Saudi millionaire, who told him that if America used chemical or nuclear weapons, "then we may retort with chemical and nuclear weapons as deterrent".

There was much more in the same vein in the al-Qa'ida leader's video appearances, mainly in tapes hand-delivered to al-Jazeera TV channel in Qatar. On 7 October, the day air strikes began, he was shown in a cave with an automatic rifle propped up nearby, speaking of the 11 September attacks. The tape appeared to have been shot a few days earlier. An American geologist said he recognised the rock strata behind him as being characteristic of one relatively small area of central Afghanistan, but if coalition bombers acted on the tip, they did not get their man. Mr bin Laden seemed to have learnt his lesson, however: in the next video, broadcast in early November, he appeared in front of a brown hessian backdrop.

That he was still alive later that month, and possibly in December, seems clear from a third video, in which he says he is speaking three months after the attacks on America. He refers to a bombing raid "several days beforehand" on the Afghan town of Khost that damaged a mosque. An American bomb accidentally hit a mosque there on 16 November.

Again there is a brown backdrop, again he is wearing a camouflage jacket. But the arch-terrorist looks gaunt and greyer, and some observers made much of the fact that he kept his left arm stiffly at his side, speculating that he might be wounded or suffering from kidney trouble. Others pointed out that in Islam the right hand, with which he made all his gestures, is considered purer than the left.

In none of the three videos does the al-Qa'ida leader admit direct responsibility for 11 September, but about the time the last one was being shot, the US authorities released another tape, said to have been found by American troops, in which he is shown laughing and joking with supporters about the attacks. More footage appeared on al-Jazeera in April: this time he is with Ayman al-Zawahiri, his closest adviser, in front of a mock rural landscape, apparently in a studio. While Mr bin Laden says nothing, his Egyptian associate hails 11 September as a triumph in the most direct al-Qa'ida claim of responsibility seen so far.

As Mr Rumsfeld pointed out, however, the video seemed to be several months old. The only tape which can definitely be dated is the one referring to the Khost bombing – and that was when American warplanes were pulverising the Tora Bora area where it is presumed to have been made.

On 10 December there was a "highly credible" intelligence report that the al-Qa'ida leader was seen with a number of his comrades outside the mouth of a cave in the region. The Americans responded by dropping a 15,000lb "daisy-cutter" bomb on the cave entrance, and the rest has been silence. Radio communications referring to the "Sheikh", the name used for Mr bin Laden by his foreign fighters, ceased. Reports that he was seen escaping the area on horseback all appear to emanate from one Afghan commander of dubious reliability.

If he is alive, then only a very small and highly secretive group knows exactly where he is. Even in Pakistan's lawless tribal territories, where warlords maintain vast fortresses with helicopter landing pads and have private armies at their command, it would be extremely difficult to prevent a whisper about the world's most wanted man from leaking out.

But in the absence of incontrovertible proof, no US official is likely to speculate that he is dead – not least because it might slow the war on terrorism, already blunted by Washington's new obsession with Iraq. And if he was buried under tons of rock in Tora Bora, such proof might never be forthcoming, turning him into what one retired American military man called "the Elvis Presley of the East".

It remains possible that Osama bin Laden will resurface to gloat over the havoc he caused a year ago. But in the week when the world remembers those terrible events, he seems most likely to remain a looming absence.