The conviction of 21 of the 28 people accused of involvement in the Madrid bombings in 2004 brings to an end one of Spain's longest, costliest and most wrenching trials, and should go some way to allaying the grief, anger and controversy over the worst terrorist atrocity in Europe since Lockerbie. Two Moroccans received symbolic sentences of more than 40,000 years in prison, and are likely to serve most of the maximum 40 years allowed under Spanish law. But one alleged mastermind was acquitted for lack of evidence, many were convicted on lesser charges, thus receiving relatively light sentences, and seven of the twenty-eight were acquitted. Some relatives of the 191 people killed and 1,841 injured in the explosions on four commuter trains were disappointed that the trial had failed to pin the blame more conclusively on many of those accused. Seven of the presumed ring-leaders blew themselves up when cornered in a flat by police three weeks after the bombing.

José Luis Rodríguez Zapatero, the Prime Minister, declared that justice had been done, but admitted that nothing could compensate for the personal loss. The verdict ought, however, to still the continuing claims by some politicians, especially in the Opposition, that Eta separatists were at the centre of the plot, even if there were informal links in the world of extremism. The six-month proceedings did not prove any direct involvement by al-Qaeda, concluding that the relatively unsophisticated operation was carried out by North African immigrants intent on “avenging” the presence of Spanish troops in Iraq. But it did underline two trends whose disturbing implications go far beyond Iraq or the borders of Spain: the virulence of extremist ideology among young alienated Muslim men, and the ease with which radicals are able to plot and carry out mass murder in imitation of al-Qaeda tactics.

Until the bombing, Spain believed itself relatively immune from Islamist extremism. It has long had good relations with the Arab world; its own history celebrates a golden age of tolerance and learning in Muslim Andalusia; and, unlike France or Britain, it has no close involvement with countries that are hotbeds of militancy such as Algeria or Pakistan. The bombings should have awoken all Europe to the nature of the threat now posed by those hijacking religion for political aims. It matters little how tolerant Western societies try to be: zealots attempting to intimidate their own communities are looking to exacerbate, not build, community relations. Even the Scandinavian tolerance of Denmark has been a pretext for extremist “outrage”.

The second grim conclusion from the Madrid atrocity is that it has given encouragement to dozens of other groups across Europe. Apart from the 7/7 bombings in London, plots have been uncovered elsewhere in Britain, in Denmark and Germany this summer. Police and intelligence agencies across Europe have smashed a succession of cells intent on terrorist carnage; but extremism shows little sign of abating. Madrid not only demonstrates the need for greater vigilance and better co-operation among security forces, but also that the ideology of hatred knows no borders and that governments, communities and faith leaders must be determined to eradicate it from schools, prisons, ghettos and the minds of those who have been infected.