By Paul Reynolds
World Affairs correspondent, BBC News website
One cannot ask the perpetrators because they are dead - though one day maybe we will be able to ask those who sent them - but the question is being asked nevertheless:
"Was Iraq the motivation for the London bombs?"
British troops participated in the invasion and occupation of Iraq
The issue has been highlighted in a report from a British think tank, Chatham House, which said that "the UK government has been conducting counter-terrorism policy 'shoulder to shoulder' with the US, not in the sense of being an equal decision-maker, but rather as a pillion passenger compelled to leave the steering to the ally in the driving seat".
The report does not say that anger over Iraq was behind the London attacks, but draws the conclusion: "There is no doubt that the situation over Iraq has posed particular difficulties for the UK, and for the wider coalition against terrorism."
Among other things, it says, Iraq "gave a boost to the al-Qaeda network's propaganda, recruitment and fundraising".
One of the report's authors, the veteran terrorism watcher Professor Paul Wilkinson of the University of St Andrews, told the BBC: "There is no doubt that Britain was on the target list before the invasion of Iraq."
But he added: "The conflict itself was a setback in our struggle against al-Qaeda."
Professor Wilkinson's remarks and his report do not directly suggest that Iraq was the prime motive or even a motive behind the London attacks, but they do describe how al-Qaeda has exploited Iraq.
Arab media watcher
I have been speaking to someone who is convinced that Iraq must have played a major role in the thinking of the four young men who took their bombs onto the London transport system.
This person, who requested anonymity for reasons of personal safety, tracks Arab media and Islamic websites, and has noted how Iraq is a major subject for discussion.
"Of course, Iraq was important. It is so frustrating to hear the government deny it.
"There is anger at the images coming out of Iraq and also from the Palestinian territories, pictures of children being killed. There is a lot of anger about. You can see this now on government television channels in places like Egypt and Syria, where discussion was limited before. Under the influence of the pan-Arab channels like al-Jazeera, these stations have had to open themselves to public debate.
More than 50 people were killed in the London bombings
"The jihadi websites are also full of Iraq. A Syrian-based one has just carried a warning that there will be further London-type attacks if all European forces do not leave Iraq in a month.
"The London bombers fit the profile of people who listen to this kind of talk, they were young and educated. But they must have been guided and directed, brainwashed if you like, and told things like: 'This is the part you can play.' Iraq would have been a part of that, probably a major part."
We know that three of the bombers spent time in Pakistan, where such "guidance" is readily available.
The argument linking Iraq to the London bombs is firmly rejected by the British government.
Prime Minister Tony Blair made this clear in his statement to Parliament on the Monday after the attacks: "It seems probable that the attack was carried out by Islamist extremist terrorists, of the kind who over recent years have been responsible for so many innocent deaths in Madrid, Bali, Saudi Arabia, Russia, Kenya, Tanzania, Pakistan, Yemen, Turkey, Egypt and Morocco, of course in New York on September 11th, but in many other countries too."
The same line was taken by Defence Secretary John Reid, who was asked about the Chatham House report.
"Terrorism is an international problem," he told the BBC, and he listed, like Mr Blair, all the places and countries in which there had been incidents - "New York, Tanzania, Kenya, Bali, Saudi Arabia, Russia, Yemen, Egypt, Pakistan, India and Turkey, all of which occurred before Iraq or in countries which opposed the intervention in Iraq."
Interestingly, the media watcher I spoke to has also noticed a backlash against al-Qaeda in the Arab media:
"Where once Osama bin Laden was seen as a hero, now there is anger at al-Qaeda as well, especially after some of the attacks in Iraq."
This could be an early sign of a trend which could yet prove damaging to the al-Qaeda network.
The Israel-based Project for the Research of Islamist Movements has reported two potentially important condemnations of the London bombs by Islamic scholars.
One, Abu Mohammed al-Maqdesi, a Jordanian, said that the bombings "might distort the true jihad".
The other, Abu Basir al-Tartusi, a Syrian who lives in London, called them "a disgraceful and shameful act, with no manhood, bravery or morality".
In reply came fervent justifications. One, in the form of an anonymous pamphlet, said: "The United Kingdom is an ally of the United States, and hence is part and parcel of the worst front of aggression the Muslims are facing in the present time. They are even worse than Pharaoh in his war against Moses."
Another mentions Iraq: "Every British citizen who voted for the members of Parliament who supported the war in Iraq should be regarded as an attacker or at least an assistant."