W HEN A MAN ran amok with two knives on November 29th, many Londoners followed official advice to “run, hide and tell”. But a few brave souls chased Usman Khan onto London Bridge, armed with a fire extinguisher and, of all things, a narwhal tusk, plucked from a display. The attacker took two lives before he was shot dead by police. The editors of Britain’s tough-on-crime newspapers—three of whom could watch events from their corner offices across the bridge—didn’t know what to make of it. Not all of the terrorist’s pursuers made for an easy moral. “One hero was a jailed murderer on day release,” the Daily Mail acknowledged.

Mr Khan’s biography poses a still trickier conundrum. It soon emerged that he had been convicted in 2012 of plotting a terrorist attack, and released early from jail last year, under supervision. He was allowed to come to central London that day to attend a conference on prison education; his victims, Jack Merritt and Saskia Jones, worked on the programme. Do these events amount to a case study in the impossibility of rehabilitating a terrorist?

The question is timely. As Islamic State’s “caliphate” crumbled, hundreds of fighters and fellow-travellers returned to European countries, including Britain. Jails in England and Wales house a churning population of 700 or so terrorism offenders and other criminals suspected of terrorist affiliations.

Boris Johnson, the prime minister, offered a simple answer. If voters backed him in the imminent election, he declared, he would make sure that all terrorists were locked up for at least 14 years, without early release. Polls suggest more than four-fifths of Britons support him. Jeremy Corbyn, the Labour leader, was ridiculed for saying that such prisoners should “not necessarily” serve their full sentences.

Yet the case presents more of a dilemma than Mr Johnson acknowledges. Such offenders invite little sympathy even from liberals. Let them out too early and you risk them re-offending. About one in ten convicted terrorists in Britain goes on to commit another terrorism-related offence. This is lower than the overall re-offending rate—29%—but highly concerning given that such offences can range from associating with terrorists and plotting attacks to mass murder.

Yet keeping terrorists behind bars too long carries its own risks. Draconian sentences can transform nobodies into martyrs and radicalise prisoners’ relatives. Some experts point to Northern Ireland, where internment during the Troubles turned civilians against the state and hunger strikes created heroes out of inmates.

Perhaps surprisingly, the police do not always support longer sentences. Some differentiate between young men who might be caught browsing terrorist material online and hardened plotters, who have spent years immersed in extremist ideology. One senior police officer says prison is ineffective if inmates can smuggle phones inside and continue plotting their activities. Either way, inmates must be released eventually. If extra years behind bars are poorly funded and structured, they “risk making bad people worse”, says Nick Hardwick, an ex-boss of the parole board.

Battlefields of the mind

Regardless of sentence length, most criminologists favour investment in de-radicalisation, which aims to strip terrorists of their motivating ideology, or “disengagement”, which has the more modest aim of dissuading convicts from future violence, even if they retain hardline views. John Horgan, an expert on extremism at Georgia State University, reckons there are 40-50 such schemes around the world.

Most involve counselling to get to the root causes of extremist sympathies. Britain already has two such schemes: one, in prison, is voluntary; another, on release, is mandatory. Measuring their success is hard. Security considerations mean governments are reluctant to allow academic evaluations. The small numbers and lack of an available control group would anyway make it tricky to draw quantitative conclusions. Even so, Mr Horgan says, “the emerging conclusion seems to be that rehabilitation can work,” but only if prisoners are committed to changing their ways.

A qualitative assessment last year by academics judged Britain’s in-prison scheme to be working well. Most lags said it helped them understand why they offended and gave them reasons to avoid doing so. Britain’s policy of mixing jihadists with other criminals risks radicalising non-terrorists. But it also exposes terrorist convicts to alternative viewpoints. One jihadist prisoner told a researcher that “being forced to mix for once” opened his eyes.

More can be done. Hiring extra psychologists might help: Andrew Silke, a counter-terrorism expert, says there is a waiting list for the in-prison course. And more work is needed on de-radicalisation. Prison governors struggle to divert inmates from violent ideology without promoting peaceful but similarly extreme views. “You have to get into the distorted ideology to tackle it,” says an ex-prison boss, recalling debates about whether to quote statements by the Muslim Brotherhood, an Islamist group.

What is clear is that Mr Khan deceived the authorities. He took part in rehabilitation in and out of prison. And he would have been allowed to take part in the conference only because his handlers believed him to be engaged, says Mr Hardwick. Yet Mr Merritt’s father, Dave, urged politicians not to become more punitive. His son died offering prisoners the chance to redeem themselves. “What Jack would want from this is for all of us to walk through the door he has booted down,” he wrote. “That door opens up a world where we do not lock up and throw away the key.” ■