As London reels from a co-ordinated bomb attack on its transport system, questions are quickly being asked about possible connections to the wave of violence linked to al-Qaeda around the world in recent years.
If a connection is established, it would show that the group is alive and well despite being the main target of US-led global "war on terror" and the attentions of police and security forces throughout the world.
Al-Qaeda is believed to be a loosely-based network dedicated to a violent re-modelling of the Muslim world and the West.
The London attacks targeted public transport passengers
Arrests and assassinations of its alleged members and leaders have taken place in many countries, but it is difficult to say that these have significantly dented its capabilities.
In fact - given the shadowy nature of the organisation and the lack of transparency of some of the agencies fighting it - there is probably more that we don't know than we do about the current state of al-Qaeda.
Until the 11 September 2001 attacks on the United States, Afghanistan was a stronghold for al-Qaeda - whose name (meaning "the base" in Arabic) is apparently derived from the militant training camps there.
The US invasion largely put paid to that presence, though the country's former al-Qaeda-allied rulers, the Taleban, remain active in parts of the country.
Some al-Qaeda trained fighters are thought to have been dispersed - perhaps to regroup in other fields of combat, such as Saudi Arabia. Meanwhile hundreds, if not thousands, of suspects have been detained without trial at American bases around the world.
High-profile arrests have come fairly regularly - one of the big breakthroughs being the capture of Khalid Sheikh Mohammed, the suspected military leader of al-Qaeda, in Rawalpindi, Pakistan in 2003.
Madrid was the first big al-Qaeda attack on mainland Europe
More recently Abu Faraj al-Libbi, said to be a top lieutenant of the Saudi-born al-Qaeda figurehead Osama Bin Laden, was captured in Pakistan's North West Frontier Province.
But there is considerable doubt as to about whether Libbi had the international influence of Mohammed, said to be the mastermind of 9/11.
However, Libbi is accused of being behind two attempts on the life of in Pakistani President Pervez Musharraf.
Bin Laden himself, as well as his top associate, Egyptian-born Ayman al-Zawahiri, have evaded capture, despite several "near misses" reported by the authorities in Afghanistan and Pakistan.
It is not clear, though, how far they may be involved in the operations carried out around the world in their organisation's name or by militants that have in the past had links with al-Qaeda.
One measure of the inconclusive campaign against al-Qaeda-linked groups has been that attacks have continued to occur on a fairly regular basis.
The London attacks seem to echo closely in style and scale a series of recent bombings which have been blamed on or linked to the group (though it is not always possible to establish culpability beyond question and claims of responsibility cannot always been taken at face value).
Bali, Istanbul, Madrid and Casablanca have all borne the same hallmarks - multiple bombings indiscriminately targeting civilians in heavily populated areas.
The trail of destruction reached Bali in October 2003
The 2002 attack on night clubs in Bali, claiming 202 lives, was blamed on a Indonesian extremist group Jemaah Islamiah, which is allegedly linked to al-Qaeda.
In November 2003, Istanbul was hit by two co-ordinated attacks in 10 days, the first against synagogues and the second targeting UK interests.
Western Europe was struck in March 2004 with the train bombings in Madrid that left 191 dead.
There have also been multiple attacks in Saudi Arabia, East Africa, and Morocco causing extensive casualties and disruption. Pakistan, Yemen and other Indonesian targets have also been hit.
It has for some time been almost a commonplace to say that Britain was "due" an al-Qaeda strike like that on Madrid. Britain's unflinching support for the Bush administration's foreign policies, particularly in Iraq and Afghanistan, make it an obvious target for al-Qaeda.