The longest terrorism trial in the country’s history has produced a verdict that protects the public from five men who plotted methodically to murder on a massive scale. It has also revealed more than any case so far about the scale of the continuing threat posed to civilians by Islamist extremism. And it has sewn fresh anger and anguish in the minds of hundreds of people injured and bereaved by the attacks of July 7, 2005, who now have reason to believe those attacks could have been prevented.

It is, tragically, true that if Mohammad Siddique Khan and Shehzad Tanweer, the principal July 7 bombers, had been identified as more than “petty fraudsters” by officials involved in Operation Crevice, 52 innocent people who died that day could still be alive. Connections could have been made, and were not, between the fertiliser bomb plot laid bare in the Old Bailey and those who led the worst peacetime attack in British history. There must be a transparent accounting for this failure, but a full public inquiry is a luxury that we cannot now afford. Preventing the next attack is a more urgent priority, and the essential lessons to be learnt from Operation Crevice are, regrettably, already clear.

At least five separate pieces of intelligence linked Khan and Tanweer to the Operation Crevice plotters. Yet the two men were deemed peripheral to that operation, and their continued surveillance once it was over was categorised as “desirable” rather than “essential”. In practice, they were neither contacted nor watched. They were left to complete their planning for the 2005 attacks unobserved. In the process they appear to have learnt from some of the mistakes that led to the Operation Crevice arrests. They used multiple small bombs rather than half a tonne of fertiliser; backpacks rather than cars as the means of delivery; and suicide rather than remote detonation. The results were devastating.

Yesterday Paul Murphy, chairman of the Intelligence and Security Committee (ISC), stood by a report his committee issued last year that criticised the failure to identify Khan and Tanweer as potential terrorist ringleaders but found “no culpable failures” on the part of the security services. Hindsight is certainly not the only perspective from which to judge performance when the challenge is as complex as modern counter-terrorism, and dozens of successes count for little in the public mind next to a single fatal lapse. Furthermore, the head of MI5 is justified in general terms in arguing, as he did after yesterday’s verdicts, that there are limits to the number of suspects the security services can keep under surveillance “in a democratic society that values its freedoms”.

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It is estimated that continuous covert monitoring of a single terrorist suspect can require up to 50 trained personnel. With about 50 alleged plots under investigation, involving several thousand suspects, 24-hour surveillance of every new name to surface in every intercept and tip-off would require the resources of a police state. Nonetheless, it is clear that the resources available to MI5 from within its own ranks and the police were inadequate for keeping track of “desirables” as well as “essentials”; that the security services’ covert penetration of domestic jihadist cells was still inadequate nearly four years after the echoing alarm call of the September 11 attacks; that connections should have been made but were not once the names of Khan and Tanweer began to recur in separate investigations; and that the possibility of UK nationals mounting suicide attacks in their own adopted country was gravely underestimated as Whitehall sought to prioritise terrorist threats.

John Reid, the Home Secretary, has so far promised no more than a review by the ISC of its original report into the background to the July 7 attacks. This may not be enough. Survivors of those attacks deserve the most detailed explanation possible for any intelligence failures in the months preceding them, and the lifting of reporting restrictions surrounding Operation Crevice will inevitably trigger valuable debate on its lessons for future counter-terrorism operations. But the argument for a full and open-ended public inquiry is weak. Such inquiries, from Bloody Sunday onwards, have obscured as much as they have clarified. In this case, continuing reporting restrictions on separate but related terrorism investigations would sharply curtail an inquiry’s power to call witnesses and elicit new evidence. The time and money would be better spent plugging the more obvious holes in Britain’s defences against terrorism.

Funds have been earmarked for a substantial increase in the number of MI5 case officers specialising in penetrating jihadist cells within Britain, but their recruitment and training are not proceeding fast enough. New routes into the UK are being used by radicals making their way from Pakistan and Afghanistan to England, including via South Africa, where local law enforcement is preoccupied with organised crime; they must, as far as possible, be blocked. And greater efforts must be made to prevent the radicalisation of young Muslims by extremist Imams within British jails.

In the freedom of British cities and universities, the poisonous influence of preachers of hate is even harder to control. Yet here, too, a new balance must be struck between the “light touch” required to maintain trust between police and certain minorities and the more purposeful approach required for public safety.

Operation Crevice saved lives by foiling a plot to detonate 600 kilograms of ammonium nitrate. It was a triumph of policing, prosecution and jury deliberation. Yet terrorists may have learnt more from it, in the short term, than did the security services. They cannot afford to let suspects fall through the crevices again.