There is an argument that once the majority are denied their liberties in the name of security that those who wished to provoke such a reaction have won. I suspect that this adds a layer too far of complexity to the thought processes of the radicalised Chechnyans that undertook this atrocity. Yet tonight, with Boston in lockdown over a manhunt, this question will be asked again.
My only experience of a lockdown during a terrorist atrocity was the tube bombings of 2005 in London. Workers were kept in their offices to remove pressure on a public transport system in crisis; whilst emergemcy services worked to normalise London as quickly as possible. Different cities will have different operational responses to these requirements. Whilst a reaction to a bombing appears appropriate, the use of a lockdown in order to apprehend the suspects should be reviewed.
The political pressure to apprehend the perpetrators is overwhelming. The school of "something must be done" leads to extreme measures and a dangerous precedent. For once the doors are opened to particular strategies of containment, these become a potentially useful tool for police forces. Once undertaken, the temptation to use this tool again becomes easier, even though one should assess the success or failure of this strategy.
As of now, house to house searches are undertaken. The name and photo of the suspect is splashed on the media. Law enforcement personnel and the private citizenry are jumpy. In similar circumstances, an innocent Brazilian was shot on the London tube by armed police. The suspect remains at large.
The final aspect of using a lockdown strategy is that the inconvenience to the citizenry starts to outweigh the intial reaction to the atrocity. The withdrawal of the freedom to live and act, if kept over a prolonged period of time, will turn minds away from the urge to find these killers and concentrate them upon these restrictions. Moral outrage and anger is dissipated by the mundane: "I can't go out to get milk".
One has to think very carefully before undertaking lockdowns; and, if the suspect is not apprehended, a difficult decision will probably be subjected to needless criticism, because the intended objective was not achieved. In such a febrile atmosphere as this, the possibility for tragic mistakes increases.