Being on MI5’S radar is not the same as being under its microscope

There are only so many terrorist suspects that the 4,000 officers can meaningfully observe

Seven years ago, a 25-year-old university dropout was caught in Kenya trying to join the terrorists of alshabaab. He was sent back home where MI5 kept an eye on him and, for a while, monitored his every move. He was certainly shifty, making calls from telephone boxes and meeting strange men in strange places. But this seemed to be explained by his low-level drugdealing; he showed no sign of terrorist intent. After two years of fruitless surveillance, the spies gave up. A month later, he bought a cleaver and killed Drummer Lee Rigby on the streets of Woolwich.

To say that Michael Adebolajo was “known” to the security services is a gross understatement: they had interviewed him, categorised him as a top-tier terrorist risk and did everything they could to monitor him. But there were then – and are now – thousands of potential Islamic terrorists on the books of MI5. To keep so many people under such surveillance would require more of a Stasi than a Security Service. And the East Germans could easily intercept letters and phone calls: today, even teenagers can use sophisticated mobile phone apps to encrypt their gossip. A free country cannot keep watch on thousands of people.

So it’s not, really, so surprising to hear that Salman Abedi, the Manchester bomber, had been reported to MI5. The question is what the spies could have done, and whether they missed anything obvious. The picture is still being put together, and it’s not impossible that there was an egregious intelligence failure. But more likely we’re just witnessing the basic problem: how hard it is, in an era of globalisation and instant secret communication, to keep tabs on so many.

Consider the numbers. When the Twin Towers were attacked in 2001 there were just 250 people listed in MI5’S books as potential terrorists. Within seven years that had grown to 2,000 and when it hit 3,000 a few years ago the spies drew the line. Not because the threat had subsided, but because there are only so many people that the 4,000 officers of MI5 can meaningfully observe. And if there are (as we learnt yesterday) 500 active investigations, then resources will be focused on them, rather than shifty losers reported by their neighbours.

When Jonathan Evans was directorgeneral of MI5, he worked out where this was all leading. The agency had doubled in size and had a good grip of the terrorist threat, but it seemed statistically inevitable that a plot would succeed – and that the culprit would have been on an MI5 list, leading suggestions that it let the killer slip. He was also concerned that the sheer number of plots being thwarted would give a false sense of security.

He was quite right to worry. In recent months, the Home Office and others in government had taken the recent jihadi attacks (knives, cars) to mean that terrorists could no longer get hold of anything more deadly. After the Bataclan attack, some senior government figures were saying that a similar atrocity would be impossible in Britain – not just because it’s harder to smuggle guns into an island, but because such a plot would be disrupted in minutes. A suicide bomb would have to have been either smuggled into or made in Britain, requiring a degree of skill beyond most of the wretches recently dragged through the courts for terrorism offences.

This helps explain why there are troops on the streets: there’s a feeling that we’re dealing with a far more severe threat, and that our defences are weaker than hoped. The killer’s Libya links also open up a hideous prospect: perhaps this lawless country is the new Petri dish of Islamism, incubating a new strain of the jihadi virus. To some ministers, it seems that the threat has mutated, and is growing faster than the intelligence agencies’ ability to keep up.

It’s quite true that MI5 can be unpleasantly surprised. A few weeks before the 7/7 attack its internal review into Islamic terrorism concluded that there was no real threat, and no group had the capability or intent to strike Britain. The spies expanded and adapted, but so did the threat. Would-be bomb-makers didn’t have to go to terror camps to learn how to kill when the instructions were all on the internet. In Pakistan, at least, we had a government to liaise with. In Libya there are just warring tribes, and no one with whom to negotiate so much as a listening post. Al-qaeda might have dropped out of the headlines but it’s still trying to plot aircraft attacks: one of many fronts on which we now have to fight.

But if the ministers can be accused of complacency, it’s harder to pin the accusation on the spies. They see anti-terrorism as a struggle that will mean a long line of invisible successes, studded by high-profile failure. On the rare occasions that the spies speak, they try to make this point, as Andrew Parker did when he took over MI5. “Being on our radar,” he said, “does not necessarily mean being under our microscope.” In other words: if an attack happens, don’t be surprised if the perpetrator was “known” to MI5. To be a “person of interest” is to join a cast of thousands.

At the Ukip manifesto launch yesterday, Suzanne Evans said that Theresa May “must bear some responsibility” for MI5 not catching the Manchester bomber. It’s an easy claim to make, especially when feelings are running so high. But a terrorist atrocity is not proof of a failing security service, and passing new laws for the sake of it (the great politicians’ vice) will not make things better.

Tony Blair fell into this trap, proposing to detain terror suspects for 90 days without trial – something the spies never wanted. Nor are they asking for more powers now. Every society draws a balance between liberty and security, and to shift that balance after one attack would be to concede a needless defeat.

There will be an inquiry into why Salman Abedi was not apprehended after the MI5 tip-offs, but its conclusion is likely to be the same as into the Woolwich murderer: there’s only so much a modern intelligence service can realistically be expected to do. The odds are that there is no political scandal here, but instead a simple and depressing truth: that we are as safe as we’re likely to get.