Despite the failure by a jury to convict eight men accused of plotting mass murder on board transatlantic aircraft, one of Britain's biggest and most expensive terrorist investigations has yielded results as disturbing as they are important. The police operation and the trial have revealed, in terrifying detail, the extent to which violent radicalism has established a hold over a few young British Muslims, the sophistication of the plots to commit mass murder, the close links with al-Qaeda and training camps in Pakistan and the callous brutality of extremists who, in various interlinked conspiracies, have been trying to kill their compatriots in the name of a twisted version of their religion.

The failure to secure convictions on the main charge is disappointing. But in no way should this be taken as a reflection on the dedication of the police, the vigilance of the security authorities or the validity of prosecuting those who are determined to visit the horrors of terrorism on this country. Three of the defendants have been convicted of conspiracy to murder - a very serious offence that is likely to earn them lengthy prison sentences. Seven men admitted making or possessing videos threatening suicide attacks. The Crown may - and probably should - decide that the men are to face a retrial. But in any case, the likely perpetration of multiple atrocities has been forestalled. Hundreds of lives have been saved. And the ideology and methods of terrorism have been forcibly brought home to sceptics casting doubt on the continuing threat and to those mainstream Muslims playing down the dangers in their midst.

It is hard, and probably unnecessary, to pinpoint the reasons for the collapse of the main charge. It is clear that the court proceedings were less than adroit, prompting questions about why the jury was given a two-week holiday at the climax of the trial. It is also clear that the jury itself, despite the judge's public thanks, was deeply divided and returned a seemingly perverse verdict. All trials face the risk that apparently watertight evidence may not seem so overwhelming to a maverick jury. There may be lessons here for the prosecution and politicians who spoke out so forcefully when the arrests were first made.

What must be avoided is any attempt to blame the Americans. Some resentful sources suggest that the conspiracy was interrupted prematurely, at US urging, before enough evidence had been gathered. This is absurd. US citizens were a principal target. It is inevitable that law enforcement officials, working in a different system, should make the prevention of a massacre the priority. It is equally wrong to fulminate against Pakistan for the prompt arrest of a suspect, triggering the round-up of those under surveillance in Britain.

The international implications are, nevertheless, important. Pakistan's role as a haven for al-Qaeda, a training ground for terrorists and a recruiting agent for alienated Muslims in Europe has been laid bare in earlier terrorist cases, but rarely with such devastating clarity. Urging the new Government in Islamabad to prosecute the fight against Islamist extremism within Pakistan must remain a priority of Western policy. Similarly the need for ever closer co-operation among Western intelligence agencies, especially the Americans, is an obvious conclusion.

Instead of recriminations, the trial should prompt greater efforts, especially among Muslims, to combat extremism. It should not be conflated with other controversial issues - the Iraq war or the proposed 42-day detention Bill - as this confuses public perception. Nor should such trials be written off: Britain has a robust conviction rate. What it shows is that al-Qaeda is far from beaten and that Britain's security remains precarious. Vigilance remains the watchword.