PARIS (AFP) - Despite putting unprecedented security measures in place for Euro 2016, France remains deeply concerned over the extremists' ability to strike a soft target.
Millions of foreign visitors and the world's press are set to descend on the country for a month of sporting action from Friday - creating endless nightmares for its overstretched security services.
"From the point of view of preparation, we have done as much as possible. Everyone has been mobilised: police, paramilitaries, many soldiers," a senior counter-terrorism official told AFP, on condition of anonymity.
But, he said: "To be totally honest, I'm worried."
President Francois Hollande acknowledged the threat on Sunday, though he tried to put a brave face on it.
"This threat will last for a long time, unfortunately, so we must do everything to ensure that the Euro 2016 is a success," he told France Inter radio.
Hollande's government introduced a state of emergency in the wake of last year's terrorist attacks in Paris, allowing police to raid homes and place people under house arrest with minimal oversight.
But the challenge of monitoring those who have returned from Syria and Iraq, or who have snuck into Europe using false passports or with the influx of refugees, has overwhelmed the continent's security services.
"What really worries us above all is the guys who are already here in Europe - guys that are already in Germany for example, that we haven't seen arrive, that the Germans haven't found, who have stayed quiet and waited," said the official.
"We have re-established some borders, but we shouldn't dream - borders cannot be controlled." A recent incident thousands of kilometres away has further darkened the mood.
On May 13, in a town north of Baghdad, militants attacked a cafe where Real Madrid supporters regularly meet, killing 16 people with automatic weapons and grenades.
"It's like a postcard sent to Euro 2016 - a direct message," said the counter-terrorism official. "It was maybe to scare us, in which case it was successful."
And tensions were raised further on Monday, when Ukraine's security services said they had arrested a suspected far-right extremist Frenchman with an arsenal of weapons and explosives who was allegedly planning "15 terrorist strikes" before and during the tournament.
"We know that Daesh is planning new attacks and France is clearly targeted," said the head of France's DGSI domestic intelligence agency, Patrick Calvar, during a recent parliamentary session, using an Arabic acronym for the Islamic State in Iraq and Syria (ISIS).
Last week, the US State Department laid out its own concerns in black and white.
"Euro Cup stadiums, fan zones, and unaffiliated entertainment venues broadcasting the tournaments in France and across Europe represent potential targets for terrorists," it said in a statement.
France has mobilised a huge security detail of 90,000 police and security guards to protect the 10 venues hosting matches around the country.
Some of the 10,000 soldiers deployed around France since last year's deadly terrorist attacks in Paris will also be used to secure the matches.
"Our objective is for the Euro to be a big festive gathering, but we owe the French the truth. One hundred percent precaution does not mean a zero per cent risk," Interior Minister Bernard Cazeneuve said recently.
While fan zones and stadiums will be under heavy guard and surveillance, a bigger risk may exist in adjoining areas.
The attacks on Nov 13 in Paris that killed 130 people demonstrated that hitting random "soft targets" such as bars and restaurants was just as effective in generating fear.
"Nothing is simpler than organising an attack. You protect 1,000 targets, so it's the 1,001st target that will be hit," said Pascal Boniface, head of the Institute for International and Strategic Relations in Paris.
"Driving past in a car and firing on pedestrians is within the capabilities of many people," he added.
In the current climate, any minor attack or even attempted attack risks ending the football competition immediately.
"If there are any deaths, it's simple - three-quarters of foreign teams will leave France," said the counter-terrorism official.
"And if Daesh claims an attack and promises more, everyone will leave. In any case, how could we continue to have a football party while we're burying victims?"