As questioning of al-Qaeda "kingpin" Khalid Sheikh Mohammed continues, BBC security correspondent Frank Gardner looks at some of the key issues following his arrest.
Where is Khalid Sheikh Mohammed being held now?
He is believed to have been flown from Pakistan to a US detention facility at Bagram air base in Afghanistan - a secure place in which to interrogate a prized prisoner.
US interrogators are now trying to make him reveal Al-Qaeda's plans for attack.
What are the key questions for his interrogators?
They will want to know where his boss, Osama Bin Laden is - he is thought to have met him only a month ago - and what al-Qaeda's plans are for any imminent attacks on the West.
They will also want to know how al-Qaeda's previous operations were carried out, and where the perpetrators are now.
How will intelligence agencies extract information from him?
The US says it does not use torture, but it must interrogate suspects to save lives.
What they will be doing is what is called stress and duress, which is to try and break him down psychologically. He will most likely be held in total isolation.
Even if he is not harmed physically, he will be subjected to sleep deprivation. He may well have to put up with 'white noise' - a continuous irritating sound. And, he will probably be made to stand, sit, or kneel, in uncomfortable stress positions.
One report I've seen suggests that the Americans may use some sort of truth drug on him, which is painless but may help him part with the truth fairly quickly.
But isn't all this a form of torture?
Amnesty International says it is concerned that the US might use torture. It says sleep deprivation is an inhumane way of getting real information out of people.
The Americans say they do not use violent methods, but they need to get this man to talk.
I think there are a number of people in Washington who would say that if this man has information, they should get it out of him as quickly as possible in order to save thousands of lives. They say that if ever there were a time for torture, it is justified now.
How important is it that they get him to talk soon?
There is an enormous pressure of time on this. Let's assume that we believe he was arrested on Saturday morning. It is possible he was arrested earlier and we were not told about it - but the first 48 hours are absolutely vital because in that time the people whom he knows the whereabouts of, such as possibly Osama Bin Laden, will have a chance to move, and change their plans.
There is the risk that if they wait too long to get the information out of him, those people will have simply disappeared, vanished again.
Will he be taken to Guantanamo Bay?
In Guantanamo Bay in Cuba, hundreds of al-Qaeda and Taleban prisoners are still being held in legal limbo, without access to lawyers or families.
But Khalid Sheikh Mohammed may never be taken here. He is so valuable a captive he will most likely be kept in a secret location where his interrogators can work undisturbed.