How vulnerable is Europe to Paris-style attacks?
The cancellation of the football match in Hannover on Tuesday night was the latest expression of a terrorist fear that currently wracks Europe.
Coming after a long month in which major attacks were seen in Egypt's Sinai peninsula, Beirut, Baghdad, Ankara and Paris, the febrile environment has generated an understandable level of concern among people in major cities across Europe.
Yet, the reality is that people face a greater daily danger from using their cars than they do from falling foul of terrorist plots in a European capital.
While the current environment is of heightened concern given attempts by Islamic State (IS) and its affiliates to massacre innocents, the reality is that plots on the scale of Paris are a rarity.
In response, European security agencies will step up their already highly vigilant posture and move to disrupt networks at increasingly earlier stages.
Outside the norm
Terrorism in European capitals is not unheard of.
Since the attacks of 11 September 2001 in Washington and New York, there have been large-scale atrocities in Madrid, London, Moscow, Oslo and now Paris.
Yet, these events are outside the usual norm.
In contrast, cities in Africa, parts of Asia and the Middle East face such attacks on a more regular basis.
Plots in the West are often disrupted - especially large-scale ones involving big networks of individuals.
While the 7 July 2005 bombers succeeded in killing 52 people in London, at least four or five other large-scale plots with links to al-Qaeda which aimed to strike the UK between 2004 and 2006 were disrupted by authorities.
More recently, concerns had focused around the phenomenon of "lone actor" terrorism - acts undertaken by individuals who did not demonstrate any clear direction from a terrorist group or network.
Sometimes the individuals proved to be part of a known radical community, but in other cases they were unknowns who had driven themselves towards terrorist activity.
And yet, while numerous such cases were disrupted, only three people were able to murder fellow citizens - Pavlo Lapshyn stabbed Mohammed Saleem to death in Birmingham, while Michael Adebowale and Michael Adebolajo murdered Lee Rigby in Woolwich.
In most other cases, the individuals were only able to injure themselves in their attempted attacks.
Difficult to disrupt
IS had noticed this phenomenon and one of the major concerns of the past few months has been the ability of individuals in the group to inspire and instigate people in the West to launch such isolated attacks.
France had faced a number of these, including a spate of attempted murders around Christmas last year.
From the authorities' perspectives, plots are inherently harder to disrupt, given the individuals' lack of connections and links to known networks, meaning intelligence tripwires were harder to identify.
Yet at the same time, these plots also tend to be less menacing in their ability to cause mass murder.
Anders Behring Breivik was an exception to this, but he remains unique in his attacks in Norway in 2011 that left 77 people dead.
In most cases, the individuals are only able to attempt to murder one or two in their immediate periphery.
Clearly, recent events in continental Europe show that the current threat picture there is more heightened than this, but plots on the scale of the slaughter in Paris remain a rarity.
While IS is clearly a terrorist organisation that has shifted its attention from state building in its core in the sands of the Levant to causing mass murder globally, the degree to which the group is able to get such large-scale plots through European security nets remains unclear.
In the wake of the atrocities in Paris, it will become even harder for the group as authorities move to disrupt plots earlier rather than let something like this reoccur.
At the same time, the current threat picture is complicated - with hundreds of Europeans and others fighting alongside IS having absorbed the groups ideology.
It is unclear how many more plotters will need to be stopped and for how long Europe will face this menace.
Raffaello Pantucci is Director of International Security Studies at Rusi in London. His research focuses on counter-terrorism and he is the author of We Love Death As You Love Life: Britain's Suburban Terrorists. Follow him @raffpantucci