BBC News Online's Tarik Kafala looks at the evidence for claims that al-Qaeda was behind the Istanbul attacks.
Pointing the finger at al-Qaeda may be an over simplification
Can we be sure that al-Qaeda was behind the latest attacks?
Not until the individuals behind the attacks are discovered and their personal histories told. Turkish officials have said two men involved in the Istanbul synagogue attacks last week had visited Afghanistan. This does not mean that al-Qaeda was behind the bombings, just that attackers may have had some link to al-Qaeda in the past.
The attacks have all the hallmarks of al-Qaeda - simultaneous suicide bombings against soft targets that represent some general idea of Western influence in the Islamic world.
The synagogue bombings and Thursday's attacks appear to have been claimed by two groups. The first, a Turkish group known as the Islamic Great Eastern Raiders' Front (IBDA-C), said it as behind the attacks in association with al-Qaeda.
The second group is the Abu Hafz al-Masri Brigades, named after al-Qaeda's former military commander. There is some scepticism about claims made by this second group. It was reported to have said it was behind the power cuts which struck London and the US in August.
Is al-Qaeda a single organisation with particular strategies and aims?
Security and terrorism experts increasingly argue that al-Qaeda is no longer anything like a single organisation run by a central leadership grouped around Osama Bin Laden. Al-Qaeda can be more accurately described as an ideology or a very loosely bound movement.
Recent attacks across the world, in Indonesia, Saudi Arabia, Morocco and Turkey, appear to have been carried out by groups with specific local or regional concerns, as well as more general aims centred on armed jihad against the United States and its allies. The groups may or may not have concrete organisational or financial links with al-Qaeda.
What they definitely share with al-Qaeda is some of its ideas and methods. Some of the regional groups are keen to associate themselves with al-Qaeda because they wish to tap into wider ideas of anti-American Islamic militancy. In certain cases, members of the regional groups may have trained in al-Qaeda's camps before the war in Afghanistan.
What are the aims of al-Qaeda and the groups affiliated or sympathetic to it?
Groups which may have links with al-Qaeda tend to have regional concerns. In Saudi Arabia the aim seems to be to overthrow the royal family. In Indonesia, the Jemaah Islamiah wants to establish an Islamic state in the region. Turkish Foreign Minister Abdullah Gul has said terrorists in his country have a problem with Turkey as a modern, democratic Muslim state.
In most cases targets with Western or US links are chosen because the attacks are partly an attempt to undermine people or organisations that are pro-Western or secular.
Al-Qaeda's central stated aim is to "launch a guerrilla war against American forces and expel the infidels from the Arabian Peninsula", according to Osama Bin Laden.
Can al-Qaeda be defeated militarily?
A widespread criticism of the "war on terror" is that "terrorism has no address and can't be bombed". A purely military approach cannot defeat al-Qaeda.
That said, the US approach to the terrorist threat is a lot broader than the wars in Afghanistan and Iraq. The US has forged political and security alliances, shared intelligence broadly and sought to strangle al-Qaeda's finances, among a whole range of other strategies.
Alongside this the US has attempted to promote the ideas of democracy and freedom that are meant to draw Muslims away from Islamic extremism and make them more sympathetic to the US and its values. This has been far less successful, largely because of anger at US backing for Israel.
If the US is able settle the security situation in Iraq and establish a stable, democratic and prosperous new Iraq, its case will be greatly strengthened.
What are the implications for the UK now it seems to be in the firing line?
Britain is already deeply committed, materially and politically, to the American approach to the "war on terror". The Istanbul attacks are likely to increase this. The signs on the US president's state visit to Britain are that the US-UK special relationship is stronger than ever.
The UK was always a likely target for terrorist attacks because of its closeness to the US. Security in Britain is already massively heightened. Terrorism specialists say that these security precautions are excellent. It is though part of the nature of terrorism that security is never 100%.