France is entering three days of mourning.  The President of the French Republic Francois Hollande has described the attacks as "an act of war against France".  At such times normal politics is usually suspended.  Yet the interlude will not disguise the fact that the bombings and shootings will have far-reaching political consequences.  The French president says the attacks were "planned outside France".  The Greek authorities say at least one of the attackers may have passed through the island of Leros with a group of 69 refugees. The man, apparently, registered in Greece and had his fingerprints taken.  It is too soon to know whether a passport, registered in Greece, found at the scene of one of the attacks can definitely be matched to one of the attackers.  But the Serbian Interior Ministry says that the holder of the Syrian passport crossed into Serbia on 7 October and sought asylum.  If the link is established the refugee crisis will take on a new dimension.  It can already be detected in Poland. The incoming Minister for European Affairs Konrad Szymanski said "we will accept refugees only if we have security guarantees".  Under the scheme to re-locate refugees, Poland was to take 4,500. That now appears in doubt.  Another Polish minister seemed to challenge the German approach to welcoming refugees when he said "we have to be aware that we were wrong, too naive and idealistic."  The President of the European Commission Jean-Claude Juncker warned on Sunday against giving in to what he called "base reactions" over the refugee crisis.  Any proof that the refugee routes have been used by terrorists will pose further problems for German Chancellor Angela Merkel. She has already seen a significant drop in her poll ratings and is under pressure to close the German borders which would be a political defeat for her.  This weekend she was comparing the crisis to the challenges of German re-unification. But uniting the German people is very different from accepting maybe a million refugees from different cultures.  The head of the BfV, the German domestic intelligence agency, Hans-Georg Maassen, said that "we observe that Islamists are specifically approaching refugees in the reception centres. We already know of more than 100 cases".  In the short term Angela Merkel will dig in, resistant to the pressures building around her.  She will also know that if Germany closes its borders the impact will be felt in country after country stretching back into the Western Balkans.  There would be a back-up of refugees and tensions between countries could rise dangerously. So the German chancellor, for the time being, will try to ride out a crisis where she is seen as having lost control.  Secondly, there is the impact on free movement of people as guaranteed by the Schengen agreement.  Already this core principle, this jewel in the European crown, is seen at risk.  The President of the European Council Donald Tusk said only days ago "let there be no doubt; the future of Schengen is at stake and time is running out…we must regain control of our external borders".  With the attacks in Paris that has become more urgent. Already there is a Belgian link with several men arrested near Brussels, a focus of the inquiry.  It will have to be established how the attackers and their weapons were moved across borders and whether border controls would have made a difference.  A number of countries, including Germany, have suspended the Schengen agreement.  France has now introduced temporary border controls. The determination to defend the principle of Schengen should not be underestimated but the longer countries return to border controls or fencing the more the principle of an open Europe will be challenged.  Fourthly, the so-called Islamic State (IS) has clearly changed its strategy.  It is prepared to stage an attack against any country that joins the military coalition against it. Through dramatic operations it wants to undermine the will of the European public to use force in Syria.  But if IS is willing to launch large operations inside Europe then control of the borders becomes a much more sensitive issue.  Lastly, the Paris attacks will feed into the sense of a crisis in Europe, that the borders are not secure at a time when there is a rim of conflict in Europe's neighbourhood.  Angela Merkel is gambling on reaching a deal with Turkey to take back refugees, to slow the exodus of those seeking a new life in Europe and perhaps to intercept terrorists.  Turkey - even more so after the attacks - remains the pivotal country.  Europe will be willing to make a deal with President Erdogan whatever the concerns about his authoritarian tendencies.  The attacks have deepened the sense of insecurity in Europe and politically the far-right will try to feed on this.  However in France, after the Charlie Hebdo killings, the National Front reaped no immediate dividend, but even though they are not necessarily connected, the killings in Paris have made the refugees crisis more complex and more sensitive to handle.
The BBC's chief correspondent Gavin Hewitt discusses how the recent attacks in Paris could add a new dimension to the ongoing debate around refugees.