As it happens, the Tsarnaev brothers, who carried out the attacks, seem most likely to have been radicalised by a combination of factors including online videos of radical preachers. These include Anwar al-Awlaki, the American-born extremist preacher who inspired numerous terrorists before and since being killed two years ago in Yemen by an American drone strike.

But there, and in the search around it, we come to the core of the problem. If there were such a thing as "Islamophobia", by what might it be caused? It is not only by the behaviour of violent radicals but the way in which the extremism they display comes from a radical agenda never nearly so many steps away as one might wish. In this melée Baroness Warsi, and indeed Boston, feature not as mere case studies, but as examplars of the problem.

In the days after the identification of the Tsarnaevs, media attention turned cautiously to the Islamic Society of Boston (ISB) whose mosque the brothers had attended. As it happens, it is an institution that has been connected to extremism and terrorism since its founding. But in the days after the arrests a good news story came out. An unnamed witness at the mosque said that in the months before the bombings the elder Tsarnaev brother had twice been challenged by other worshippers at the mosque, for interrupting sermons with anti-American rhetoric. But this much-desired example of a mosque actually dealing with a radical did not last.

For then came the discovery — leaked onto the blogosphere — of a different story. The leaked email had been sent by the ISB to its congregation in the wake of the discovery of the bombers' identities. The mosque had warned them about co-operation with US law-enforcement.

"We have been informed that the FBI may be starting to question some of the community members about the two suspects," the email read. "Insha'Allah we want to help as much as we can, but of course not put ourselves at risk either." The ISB told its members to get in touch with the American Civil Liberties Union (ACLU) and the mosque "for other resources" if approached by law enforcement. What is any normal American — including any normal Muslim American — to make of such a revelation? Surely to wonder with everybody else what the risk could be to mosque members in speaking with the FBI?