What has happened to Osama Bin Laden? Is he dead? On the run? Or readying for new attacks on the US?
Mystery surrounds the fate and whereabouts of Osama Bin Laden
These are questions that have been asked almost constantly for the past three years, but have a particular resonance now.
On the two previous anniversaries of the 11 September attacks, Bin Laden made some kind of appearance.
In 2003, it was video footage of him walking in the mountains, although it was unclear when it was filmed.
In 2002, his voice was heard on a video praising those who had carried out the attacks on New York and Washington.
But so far this September, we have yet to hear from him.
The last statement which is believed to have come from Bin Laden himself was the audio message of 15 April.
In it, he gave Europe three months to consider an offer of a truce if it committed to not attacking Muslims or interfering in their affairs.
There have been no major attacks in Europe since the end of that deadline.
A video did appear from Ayman al-Zawahri, al-Qaeda's deputy leader, on 9 September which was designed to boost the morale of jihadis by playing up America's problems in Iraq and Afghanistan.
Of course, Bin Laden's non-appearance cannot be read in itself as a sign that he has died or is no longer hard at work - he could well be biding his time waiting to appear to gloat after a major attack on the US, an attack that American officials say could come in the next few months around election time.
However, his non-appearance still raises important questions about what exactly his role is in al-Qaeda and the broader global jihadist insurgency.
Pulling the strings?
Since 2001, there has been a tendency from both the media and some politicians to personalise the conflict, setting up Osama Bin Laden as an all-powerful puppet master, pulling the strings of a global network that reports to him, and is responsible for every attack.
Before September 2001, there was some evidence to back up that view.
In the case of the 11 September plot itself, the recently released US independent report makes clear that Bin Laden was involved in the plot to a greater level than thought before, actually making changes to details of the plans and intervening over when the attacks should take place.
Iraq has become the new battleground for anti-US militants
But things have changed over the past few years. The US has been saying that three-quarters of al-Qaeda's known leadership has been captured or killed.
The core of its leadership is undoubtedly under pressure and having to take extreme care over movements and communications, limiting its ability to direct plans - video messages can provide many clues on location which may be one reason they do not appear so often.
But the problem is that Bin Laden's jihadist ideology has spread like a virus around the world.
Mystique of invincibility
Many of the attacks, like those in Madrid and Jakarta this year, almost certainly were not centrally directed by al-Qaeda but launched by groups and individuals who follow Bin Laden's ideological agenda.
Bin Laden is unlikely to have directed the Madrid train attacks
In some cases, individuals and networks have been inspired by Bin Laden but may never have trained at his camps in Afghanistan.
In other cases, existing regional groups have adopted al-Qaeda's narrative of needing to target the "far enemy" of the US and western interests rather than just the local state which they have always been fighting.
New leaders are emerging on the local level, sometimes with limited links to Bin Laden, who can build their own networks and stage their own attacks.
The best example of this is Abu Musab Al-Zarqawi in Iraq who has never been formally part of al-Qaeda itself, and has built his own parallel network.
There has, of course, been speculation that there will be particularly strenuous efforts to find Bin Laden in the run-up to the US presidential election in November.
Personification of the threat
On one level, this would be a huge morale boost for the US and a blow to al-Qaeda's supporters, undermining Bin Laden's mystique as some invincible leader.
The US assault on Afghanistan has forced al-Qaeda into hiding
But some analysts believe Bin Laden's departure from the scene would have only a limited impact on the overall level of violence.
"In reality it's going to make no difference whatsoever, because his ideology has now infected individuals and groups right across the globe," says MJ Gohel of the Asia-Pacific Foundation.
"From the very beginning, there has been a mistake in understanding the terrorist threat we face. We're not dealing here with a single organisation, a single command and control structure. We're dealing with an ideology and an ideology is very difficult to defeat."
Getting rid of Bin Laden might simply reveal the complex reality of local conflicts, with their roots in specific ethnic, regional issues, and the ideology will live on after its propagator has passed from the scene.
With the personification of the threat gone, it may also make it harder for American politicians to generate the kind of domestic support needed to continue the fight.
Others disagree, arguing that even though he is unlikely to have been involved in planning and executing most of the attacks since 11 September, 2001, Bin Laden still plays a central role in communicating broad strategic objectives which others will then carry out in their own time and manner.
"There's a conventional wisdom that it doesn't matter if Osama Bin Laden is captured or killed, which I think is erroneous," argues Peter Bergen - one of the few western journalists to have interviewed the al-Qaeda leader.
"Both Osama Bin Laden and Ayman al-Zawahri continue to influence the broader al-Qaeda movement, giving them broad strategic guidance via the medium of these audio tapes.
"For instance Bin Laden called for attacks on members of the coalition in Iraq, there were then attacks on Italian police barracks in southern Iraq; there were attacks on the British bank and consulate in Turkey, there was the attack in Madrid."
Capturing or killing Bin Laden remains a key goal for the United States. Doing so would undoubtedly be a major morale booster and would also have some impact on the direction of the struggle - but it almost certainly would not end it.