BBC security correspondent Frank Gardner examines the significance of the arrest of suspected al-Qaeda kingpin Ali Abdul Rahman al-Ghamdi.
How will the arrest affect al-Qaeda's operations in Saudi Arabia?
It is certainly a big blow. Even if he wasn't al-Qaeda's top person in the country, he was certainly in the top three.
He is believed to be the link man between the Saudi cells and what remains of al-Qaeda's fugitive leadership which is hiding out in Afghanistan and Pakistan.
Ghamdi: One of the top al-Qaeda operatives in Saudi Arabia
He is somebody who is alleged to have extensive experience with al-Qaeda, fighting in Afghanistan in Tora Bora, some sources even say in Chechnya as well.
He is quite young - in his 20s - so he wouldn't have had experience of fighting the Soviets, but he would certainly have knowledge of a lot of contacts in al-Qaeda if that is true.
What does this mean for the wider war on terror, and US-Saudi relations?
This has been described by US security officials as a very significant development in the war on terror and a major blow to al-Qaeda operations in Saudi Arabia.
The Americans have been dissatisfied for some time with Saudi Arabia's efforts in combating Islamic extremism and also in cracking down on suspected terrorists.
The Saudis have made some moves, but up until recently they haven't been enough to keep Washington happy.
That changed on 12 May when Saudi Arabia was attacked - for the first time in many years - by Islamic extremists, and we have seen a massive crackdown since then.
It is believed there are about 100 al-Qaeda operatives inside Saudi Arabia and the Saudi authorities say they have rounded up about 50 of them.
What they would like to see is the remaining 50 hand themselves in in the way that apparently Mr Ghamdi has done.
What do we know about how Ghamdi was arrested?
The reports out of Saudi Arabia suggest that Mr Ghamdi handed himself in in Jeddah on Thursday morning after dawn prayers, at the house of the deputy interior minister.
I understand that it was Mr Ghamdi's father who persuaded him to hand himself over.
This would be quite extraordinary. No al-Qaeda suspect willingly gives themselves up, particularly in somewhere like Saudi Arabia where if he is found guilty of having anything to do with these bombings, normally you would expect him to be beheaded.
There has been a crackdown since the Riyadh bombings
But it may be that they have cut some kind of deal, that he will be tried under Sharia law and that some kind of plea bargain may have been negotiated. Certainly the investigators in Saudi Arabia will want to find out a lot of names and places, addresses and contacts from him.
How deep is al-Qaeda's support in Saudi Arabia, and how will the arrest affect it?
Al-Qaeda's support in Saudi Arabia fluctuates. It depends on the unpopularity of the United States, which is generally fairly unpopular in the Middle East, but also on what al-Qaeda's actions have been recently.
After 11 September 2001, al-Qaeda was relatively popular with people in Saudi Arabia.
There are even reports coming from the Saudi Government that the militants they caught had explosives inside copies of the holy Koran
That is not to say that all Saudis are helping al-Qaeda. Not at all. But they may verbally express their sympathies because Osama Bin Laden represents somebody who calls themselves a devout Muslim and is standing up for the rights of Muslims - as they see it - in the Middle East.
That though will have changed after the Riyadh bombings because this was an attack on Saudi soil in which Saudis and Muslims died - very different from what happened on 11 September which was an attack on the heart of America, which some people see as the heart of the enemy.
Plus we have got the discovery of a cell in Mecca. There were explosives there and there was a shoot out there last week where a number of militants died.
There are even reports coming from the Saudi Government that the militants they caught had explosives inside copies of the holy Koran.
If true, it would be very hard for most Muslims to accept. The Koran is a holy and respected book for more than one billion Muslims around the world - you do not tamper with it.