When a terrorist attack takes place, it is a tragedy which affects everyone in the worst possible way. It is a disaster for the people who die or are injured in the attacks as well as for their loved ones. It creates a general atmosphere of fear and insecurity for all members of society and leads to preventative measures being enacted that erode people's civil liberties more and more. When the attackers attribute themselves to a certain religious or ethnic minority in that society, it leads to a societal backlash against the innocent members of that minority group, as we have seen with Muslim communities in the West when terrorist attacks take are carried out by criminals abusing Islam's good name.
Muslims, just like everyone else living in society, must do what they can to prevent terrorist attacks from taking place. In many countries, Muslims are being called on by their governments to report Muslim "extremists" in their midst who are likely to engage in terrorism. This puts Muslims in a difficult situation. Who are the extremists? When does an extremist become a terrorist? Should Muslims report their co-religionists to the authorities? If yes, when should they do so?
The first question that must be dealt with is the ambiguous concept of "extremism" and when it becomes a problem. This is a difficult question, since people have widely varying definitions of what constitutes "extremism" and "terrorism". We will need a have a practical understanding of this concept – without trying to form a definitive definition of terrorism – before addressing the question of reporting people to the authorities.
There are Muslims in the East and the West who choose to follow an austere and harsh – some might say "extreme" – approach to religious matters, both in their worship and in their interactions with others in society. Some of these people go further and try to insist that other Muslims conform to their ideas about religion, especially in matters of worship. In spite of all that, they do not subscribe to the "jihâdî" ideas of resorting to violence against their non-Muslim neighbors. They do not wish for any violence to take place at present nor do they do not plan to perpetrate acts of violence against anyone at any time in the future. If they were to hear someone suggest to them perpetrating acts of violence against others, they would be deeply offended and disgusted by the idea.
Such people should certainly not be reported to the authorities since they do not pose a threat to anyone in society, nor do they present a threat to the government or to national security.
Then there are others who follow an approach of excommunicating from the faith people who disagree with them – often referred to as a "takfîrî" ideology – and who adopt a "jihâdî" approach to non-Muslims and to anyone else they define as being outside of the faith. These people legitimate violence against non-Muslims and make plans to carry out various violent acts in society. They often actively encourage other Muslims to subscribe to their deviant views and to participate in carrying out acts of destruction and violence. They establish secret organizations and associations to further their criminal objectives. Their plots only surface once they carry out their crimes in civil society.
It is permissible to report such people to the authorities in a non-Muslim country. This is necessary to prevent their ignoble activities form causing genuine harm to innocent people in society; people – whether Muslim or non-Muslim – whose lives are sacrosanct in Islam. The fact that the criminals in question are Muslims does not prevent us from reporting them to the non-Muslim authorities, especially when there is no other effective and practical way to advise those Muslims and to prevent them from perpetrating serious violent and destructive criminal acts like the detonation of bombs in public areas.
There can be no doubt that turning such people in to the authorities causes those people harm. However, this is to prevent a far greater harm. It is a general axiom of Islamic Law that a lesser harm is to be borne if it is necessary for the prevention of a greater harm. We may not like the idea that the non-Muslim authorities will be dealing with these Muslims, and that those Muslims could face criminal proceedings and even punishment. However, this is something unavoidable in order to prevent tragedies of a much greater scale and magnitude. If we fail to report those who are planning to perpetrate acts of violence in our societies, then many innocent people – Muslims and non-Muslims alike – will pay the price.
At the same time, it must be emphasized that no one should report anyone else to the authorities without first ascertaining that the person truly poses a threat to society. People should not go around reporting others on the basis of mere suspicions that are as likely to be as false as they are to be true. Only those who are confirmed to be harboring a violent intent should be reported to the authorities. Also, if there is a chance of convincing people who harbor such tendencies to mend their ways, this is always the better approach. However, if there is no other way to protect society from a real threat that a person's activities presents, then it is clear that the person must be reported to the authorities.
And Allah knows best.