In the wake of any attack, the same questions are asked. In the wake of those last week, they are more urgent than ever.
Answering them is crucial to determining the level and nature of what is clearly an ongoing threat.
The first step is to establish what links there may be between the attackers in Paris and international organisations.
It now seems clear that two of the three known members of the network - the two Kouachi brothers who attacked Charlie Hebdo - had spent time in Yemen.
At least one had met Anwar al-Awlaqi, the Yemeni-American propagandist based in the country who played a key role in the radicalisation of a series of extremists in the US and the UK before his death in a suspected drone strike in 2011. Al-Awlaqi was a senior figure within al-Qaeda in the Arabian Peninsula organisation (AQAP), an affiliate of the main al Qaeda group.
AQAP have a long track record of trying to attack the West, and the Kouachi brothers have themselves claimed that they were acting on the group's behalf. A link here seems clear - although the degree of command and control exercised by the group does not.
The third attacker, Amedy Coulibaly, claimed allegiance to Islamic State (IS), the group which has overrun much of eastern Syria and western Iraq. Here, a connection seems less evident, though entirely feasible. His partner, Hayat Boumedienne, left France days before the attacks for, it is thought, Syria. The group has repeatedly called for attacks on France and several Frenchmen are known to be with IS, including one who is a close associate of Coulibaly.
So could IS and AQAP be co-operating?
This seems unlikely. The two organisations are deadly rivals. IS broke away from al Qaeda senior leadership and denounced them as betraying the true legacy of Osama bin Laden, the founder of the group who was killed nearly four years ago.
One possibility is that, if rivalries are bitter at the top, they have no relevance on the ground. Islamic groups today centralise decision-making but decentralise execution, allowing key individuals to work out who to recruit and how to carry out an attack themselves. The attacks last week may simply be part of this new chaotic, hybrid militancy involving some input from above, but a lot more down below.
Another possibility is that AQAP itself is splintering. The group has been under pressure for some time, with drone strikes killing many leaders and offensives by both government troops and Shiite rebels in Yemen forcing it out of territory captured earlier.
It is entirely possible that, in the constantly shifting world of jihadi groups, we are looking at a new combination of allies, perhaps one faction within AQAP establishing a relationship with IS.
What is certainly the case is that the best way for either IS or al Qaeda to win the battle for credibility within the Islamic militant movement is to pull off a spectacular attack in Europe or the US.
Both groups recognise that any targets must seem legitimate to supporters and potential supporters, however. They are thus carefully chosen.
Charlie Hebdo, the satirical magazine with a track record of publishing cartoons of the prophet Mohammed, was reviled in much of the Islamic world, let alone among extremists.
Jewish targets are also a default option and will thus be high on the danger list for the foreseeable future. They are also, on the whole, much more vulnerable than a symbolic building connected to the US or the French state, such as a military base or even an airport.
This search for legitimacy, according to their own twisted codes, explains much. Jihadist militants, few of whom have any deep understanding of their supposed faith, spend a great deal of time discussing the religious justification for their acts, referring naturally to a series of extremist interpretations of Islam's holy texts.
The seemingly esoteric debates about "offensive" or "defensive" jihad have a very real impact, explaining the apparently surprising comments from one Kouachi brother that he did not want to kill all "civilians", and Coulibaly's rationalisation of his own attack as "an eye for an eye".
These kinds of statements have been part of the extremist narrative for more than 30 years and continue to be deployed by senior militants like al-Awlaqi to intensify the radicalisation of new volunteers.
Given all of this, the failures of French intelligence would seem very significant. But if their resources have increased significantly over recent years, all European security agencies have been sorely tested by the ongoing Syrian war. More than 900 young Frenchmen have travelled to the war there, the French minister of the interior said last year, and many have returned.
At the same time there are people going - like the Kouachis - to Yemen or other conflict zones. All of them pose a potential danger. But no security service can watch them all, all of the time. It can take between 20 and 30 people to watch one individual around the clock. So decisions have to be made about who is and who is not a threat.
In this instance, someone clearly made a tragically bad call.
Similarly, warnings from other agencies, such as that reportedly given by the Algerian secret services, are frequently too vague to be of any use. Unless there are specifics, such intelligence is not "actionable" but just part of the huge flow of data constantly pouring in.
A report - or several - will now, no doubt, be commissioned and its conclusion will be that there were significant analytic and institutional failures at several levels. This will be put right, more or less.
But as the past decade or so has shown us, by the time this happens, the threat will have evolved once again, and the danger will still be there.
