Osama Bin Laden's deputy, Ayman al-Zawahri, has appeared in a video tape threatening London with more bombs and demanding that the US leave Iraq.
Ayman al-Zawahri warned the US to withdraw from Iraq
The BBC asks former CIA agent Robert Baer, who spent most of his career working in the Middle East and investigating terror, whether it is a credible message.
Q: How much influence does Zawahri wield?
When Zawahri goes public and says "conduct more attacks in London", there will be people who will attempt to do it.
It is the power of influence - as of today we have not seen anything from London's Metropolitan Police or MI5 suggesting that either Bin Laden or Zawahri were directly involved in the attacks. But still they are worried because these men have an enormous amount of influence.
Q: How much support do the two men have?
Polls show that they have a large following in Pakistan and Saudi Arabia and you can believe that the faithful will take this seriously and attempt to undertake operations.
There is a view that Bin Laden may have gone out of bounds on 11 September and in other attacks, but he still defends the Muslim community. There is popular support for him - he is not an outlaw in that sense.
Q: Given the pressures on al-Qaeda, could it have been directly responsible for the London attacks?
I doubt it. They have gone off the phone - they used to use satellite phones and cell phones but they know now that they are all being listened to, and it would be a very complicated chain of command if they were sending these things verbally.
I think it is just more likely that these people [the London bombers] are taking their orders off Jazeera television or off the internet, as well as getting their technology.
Q: Is al-Qaeda an organisation in the conventional sense?
I think it was until 11 September, 2001, but since then it has been more or less beheaded.
The organisation is diffused - a lot of members are in hiding. But it is an idea and it does have the ability to command the faithful and this is what should worry people.
Q: If al-Qaeda is viewed as an idea, rather than as a defined organisation, can it be destroyed?
No - you have to remove the causes for the hate and that would include the war in Iraq - saying otherwise is just silly.
It would have to do some sort of fair settlement with the Palestinians and Afghanistan as well.