It may run counter to common wisdom, but the jihadist terrorist threat to Western societies remains marginal. The number of casualties inflicted by terrorist acts remains low, and the economic damage caused by terrorist acts or, indirectly, by enhanced spending on security efforts, can be absorbed by national economies with relative ease.

The ideological appeal of jihadist terrorism is as weak as ever, and even in most parts of the Muslim world, radical Islam remains deeply unpopular. Jihadist terrorism can instil fear and cause serious disruptions within Western societies. Yet despite worrisome headlines about car bombs in London, suicide attacks in Iraq, and heavy fighting between NATO forces and Taliban in Afghanistan, jihadist terrorists are nowhere near any kind of victory.

However, this could change significantly if jihadist terrorism chose to ‘go nuclear’.

A 10-kiloton nuclear device detonating at New York’s Times Square would destroy all buildings within a range of 500 metres, and instantly kill half a million people. Fire, debris and long-term radiation effects would drive the death toll to well over one million. The economic, and above all, psychological impact of such an act would far exceed that of ‘9/11’ or the anthrax letters that paralyzed Washington in the following weeks. A ‘nuclear 9/11’ would leave Western societies demoralized, while at the same time giving a boost to jihadist worldwide.

But how realistic is such a scenario? The fact that various terrorist groups have been seeking to acquire nuclear material and know-how is well documented; but could terrorists plausibly acquire or even build a nuclear device?

Equally importantly, how would a ‘nuclear 9/11’ square with Islam, a religion that prohibits the killing of innocents?

Finally, given some terrorists’ obvious quest for the nuclear option, why has such an attack not yet happened?

Nuclear terrorism can have many faces. The simplest option would be the release of radioactive material in densely populated areas.

In 1987, near the Brazilian city of Goiania, some scrap metal scavengers broke into an abandoned radiological hospital and removed a container with highly radioactive caesium. In the weeks that followed, several people died, many more suffered serious health problems, and several contaminated buildings had to be dismantled.

The Goiania incident was an accident, not a terrorist act. But it indicates what could happen if terrorists had radioactive material at their disposal. Given the significant amount of such material that has gone missing from hospitals, power plants, etc., some experts believe that certain terrorist groups may already possess enough of it to stage an attack of that kind.

Another plausible option would be an attack on a nuclear power plant. Since 1972, when the hijackers of a United States (US) passenger plane threatened to crash the airliner into a nuclear reactor in Tennessee, this scenario has acquired considerable credibility. And 9/11 further highlighted this risk.

Another scenario is the use of a so-called radiological ‘dirty bomb’, in which non-fissile but highly radioactive material is mixed with a conventional explosive, such as TNT. The detonation disperses the radioactive material, causing relatively few casualties, but contaminating wide areas. If such a bomb was exploded in a major city, the long-term radiation effects would make entire parts of that city uninhabitable, causing enormous economic damage. For this reason, the radiological bomb has been labelled a “weapon of mass disruption” rather than mass destruction.

The design challenges of such a device are considered to be modest. Moreover, in March 2002 US authorities arrested a man suspected of working on such designs for ial Qaida/i. And in November 2007 Slovak authorities seized uranium powder that was enriched enough for use in a radiological bomb.

All this has led many experts to conclude that for those bent on waging ‘nuclear jihad’, the ‘dirty bomb’ could well be the most readily accessible option.

Tabloid articles about building a nuclear device from blueprints available on the internet are way off the mark. Up to now, the capability to build a nuclear device has been widely believed to rest with state actors only. The elaborate technical infrastructure required is simply not available to non-state actors, no matter how much money they might have at their disposal. Thus, even though al Qaida may have recruited several experienced nuclear physicists, it remains doubtful whether they could build a fission bomb.

Even if terrorists were to obtain a fully fledged weapon, they would have to overcome its secure command and control features. Moreover, it appears that some of al-Qaida’s attempts to obtain nuclear material on the ‘black market’ failed precisely because the customers’ limited nuclear expertise made them easy victims for swindlers.

Such difficulties might be overcome, however, if terrorists could acquire a functional nuclear weapon from a like-minded regime.

This scenario may strike many observers as highly unlikely, yet it can no longer be entirely dismissed. Cash-strapped North Korea has already hinted that it might consider the sale of nuclear-capable missiles a legitimate source of income.

And 2004 reports about the commercial nuclear smuggling network run by the entrepreneurial Pakistani metallurgist A. Q. Khan revealed the existence of a secret market, unconstrained by political or ideological inhibitions. Khan reportedly supplied Libya, North Korea, Iran and other customers with weapons designs and components rather than fully fledged weapon systems, yet future dealers might go even further.

Finally, the debate about a possible ‘Talibanization’ of Pakistan, where religious radicals might take over the state and its nuclear arsenal, has raised the spectre of a jihadist nuclear power.

Terrorist groups such as al-Qaida claim to act in the name of Islam. As this religion lacks any authoritative expositor of doctrine, it is subject to a wide range of different – sometimes outright contradictory – interpretations by individual scholars and clergymen.

That said, however, it has been widely understood that, not unlike Christian’s ‘just war’ doctrine, Islam does not allow for indiscriminate attacks on civilians. Accordingly, many religious references by Osama bin Laden, for example his 1998 statement that acquiring nuclear and chemical weapons was a “religious duty” for Muslims, may appear like entirely self-serving interpretations of Islam, intended solely for justifying personal aims.

It would be a mistake, however, to dismiss such statements as mere religious window-dressing. For many years certain radical clerics have been reinterpreting this religion with the obvious aim of tearing down its inherent moral and legal barriers to indiscriminate violence.

A first significant step in this reinterpretation already occurred in the 1980s with the suicide attack on the US Marine barracks in Lebanon and the upsurge in Palestinian suicide bombings. While Islam does not approve of suicide, and thus implicitly opposes suicide bombings, numerous clerics started to argue that suicide bombers were actually “martyrs” and acting in accordance with God’s aims. In this interpretation, even the killing of innocent fellow Muslims can be tolerated, for they become martyrs themselves.

This tendency to glorify suicide bombing by re-branding the perpetrators has extended far beyond the Israeli-Palestinian context. If killing innocent Israelis is permissible because Israel is occupying Palestine, the United States, whose soldiers are deployed in several Muslim countries, can be subjected to similar reprisals. Such was the logic of Osama bin Laden in justifying his attack on the US on ‘9/11’. Moreover, because Americans collectively elect their leadership, the entire US population could be held responsible for the sins of its government.

Since ‘9/11’, radical clerics have extended this logic even further. In his 2003 work, ‘A Treatise on the Law of the Weapons of Mass Destruction Against the Unbelievers’, one Saudi radical scholar argued that since roughly ten million Muslims had been killed by Americans, killing that same number of Americans was permissible, including through the use of weapons of mass destruction.

Such a fatwa (an Islamic scholar’s or clergyman’s analysis on interpreting Islamic law) effectively provides a religious blank cheque for mass murder. Against this backdrop, statements by some Islamists, including religiously fervent physicists who maintain that Pakistan’s nuclear weapons ‘belong to all Muslims’, acquire a new, worrying significance.

If al Qaida and other terrorist groups are still making significant efforts to acquire nuclear materials, and if the religious justification for nuclear terrorism seems to have become more explicit within certain strands of Islamist fundamentalism, why has this kind of attack not yet been conducted? There are several possible explanations.

The most straightforward explanation centres on the immense technical difficulties of orchestrating such an attack. In this view, al Qaida simply has been unable to master the technical challenges involved in such a plot. As of today, no state appears to have supplied al Qaida with the elaborate infrastructure necessary to ‘go nuclear’, let alone any fully working devices.

Another view holds that the enormous loss of life caused by such an attack would undermine rather than advance the terrorists' cause. According to this view, a terrorist act of such a scale would alienate large segments of al-Qaida’s own constituency, which accepts terrorist methods in principle, yet would not condone the wanton destruction of a ‘nuclear 9/11’.

A third explanation is that some of al-Qaida’s efforts may have been disrupted through counter-terrorism efforts. Indeed, it appears that since ‘9/11’ several planned attacks involving the use of weapons of mass destruction have been thwarted.

Moreover, the international intervention in Afghanistan has effectively denied al Qaida its major home base and has forced it to disperse, thereby making any concerted planning of a nuclear attack far more difficult. The national and collective measures taken by many governments, such as intelligence cooperation, enhanced container security, uncovering nuclear smuggling networks, securing ‘loose nukes’ from the former Soviet Union, and draining terrorist financing networks, may have further degraded the ability of terrorists to launch a nuclear attack.

Yet another reason might be deterrence. While much has been made of the claim that suicidal terrorists can not be deterred, states that sponsor terrorism are likely to remain susceptible to threats of retaliation. Since terrorist cells require territory on which to train and from which to operate, a threat against any country willing to serve as a ‘host’ might have a restraining influence on the kinds of activities it may allow its ‘guests’ to undertake.

This connection between non-state and state actors lies at the heart of French, British and US statements about the role of their nuclear forces in deterring state-sponsored terrorism. Coupled with improved ‘nuclear forensics’, i.e. the technical ability to trace an attack back to its sources, such statements may indeed have a deterrence value against states that provide a safe haven for terrorists.

The ongoing efforts by jihadist terrorists to acquire weapons of mass destruction, including nuclear and radiological weapons, continue to be among the key challenges to Western security. While the terrorist use of a nuclear bomb remains unlikely due to serious technical obstacles, and while international cooperation on counter-terrorism and non-proliferation may have made acts of nuclear terrorism more difficult to carry out, the use of a radiological ‘dirty bomb’ is an eventuality that Western societies need to be prepared for. Above all, this may require greater investments in national and international capabilities for detecting the traffic of certain materials as well as for alleviating the consequences of an attack.

Equally worrisome are tendencies among radical Muslim clerics to propound interpretations of Islam that justify the use of weapons of mass destruction against civilians. However, arresting these fateful tendencies by denying jihadist terrorists their religious justifications is a challenge that cannot be met primarily by the West. Speaking out and acting against this brutalisation of Islam is first and foremost a challenge for Islam itself.