A security officer guards the entrance of a hotel in Alfortville, outside Paris, Tuesday, Nov. 17, 2015. http://syndication.ap.org/AP.Distro.ContentBroker2/ContentBroker.aspx?contentid=b9213ab0abe8fe33870f6a706700235b&iid=742edccd3727439d8af08a5e0f0b2558&rsn=0&recordid=742edccd3727439d8af08a5e0f0b2558&filingId=d01365b6647b4d77966a8c7c13c72065&role=Preview&reldt=2015-11-17T14:31:27&media=Photo&sz=&dest=ak&trF=MEU101&ofn=France%2bParis%2bAttacks.JPEG&fmt=jpg&relativeUrl=jpg/2015/201511/17/b9213ab0abe8fe33870f6a706700235b.jpg&s3Key=preview.jpg&authToken=eNoti0EOhCAQBF%2bEaRwY9UCyXwHFZA4rRiTrYR6%2fbLKX7kNVaX4Ck5%2bJnWMQAIfF6yohLaOlmGBiyrPZM5GZJ%2byG4wSegJF80iJbz%2bFJWw1rOe5LUrvLVV%2bpVTly7VNly9ewlrc26bIFk56f8AfWQ%2bUM7IYOBvt7678duC38 More than 1,200 Europeans who joined Islamic extremists in Syria and Iraq have returned home in the past two years, an Associated Press count shows. Many have been jailed but others — absorbed into the underbelly of some of the continent's biggest cities — have thrived with impunity.

All five Frenchmen linked to Friday's attacks in Paris — four strapped with suicide vests and the fifth on the run — are among them, according to officials linked to the investigation, redoubling fears that the returnees form a pool of potential terror attackers. Many remain off the radar, and France's Interior Minister Bernard Cazeneuve acknowledged Tuesday that "the majority of those who were involved in this attack were unknown to our services."

The Belgian believed to have masterminded the Paris attacks bragged about his ability to return home from Syria, saying an ID check by police raised no flags. Two of the Frenchman responsible for the rock concert bloodbath had apparently done the same back and forth unnoticed, despite having files linking them to terrorism and Islamic radicalism.

France has the uncomfortable distinction of being Europe's leading exporter of jihadis — nearly 1,600 out of a continental total of over 5,000, according to government figures. And despite the government's promises after the Charlie Hebdo attacks in January to block and prevent citizens from leaving for the war zone, the pace of departures has remained essentially unchanged.

An Associated Press analysis of government figures puts it at about 13 a month in the first 10 months of the year compared to an average of 12 a month in 2014.

Neighboring Belgium has sent more young men and women per capita than anywhere in the West. And the two groups of foreign fighters are bound together by a common tongue and nearly as often a common background, often living in the same compounds and entering the same combat units.

French protesters supporting Charlie Hebdo in the wake of an attack on the magazine's headquarters. Olivier Ortelpa / Wikimedia Commons

Both countries have paid the price in blood: last week's attacks in central Paris left at least 129 people dead; the coordinated assaults in January on the Charlie Hebdo newspaper and a kosher supermarket killed 17; the attack against a Jewish museum in Brussels that killed four last year. All the attacks were carried out by Frenchmen with close links to extremists abroad and all too often Brussels, and neighboring Molenbeek, on their itinerary.

"With the conflicts in Syria and Iraq in particular, there has been a radicalization that we have never seen before," said Molenbeek Mayor Francoise Schepmans in her office Monday. And the government knows the neighborhood has long had trouble to contain it, as too many terror cases show.

"I see there is nearly always a Molenbeek link. There is a gigantic problem," said Belgian Prime Minister Charles Michel.

The French government's most recent figures put the number of returnees to that country at 250, but the number is clearly far higher. With the attacks on Paris and the coordinated assaults in January, French citizens have become both the leading killers among European extremists and its primary victims.

ISIS militants seen in a recruitment video. Youtube/AlHayat TV French officials estimate about 520 citizens are currently with extremists in Iraq and Syria, a number that has climbed steadily despite government promises to make blocking departures a priority. The numbers of departures exploded by 2014 — young people spurred in part by the chemical weapons attack that killed as many as 1,400 Syrians in August 2013 who said they wanted to help Syrian civilians and fight President Bashar Assad. The vast majority ended up with the Islamic State group.

Look beyond France, and there is a sense of the scale of the problem, and of the rising alarm of European intelligence officials. And because of Europe's open borders, returns appear to be nearly as fluid as departures.

"Syria has become the biggest factory of terrorists that the world has ever seen," French President Francois Hollande said Monday.

According to an AP count, Britain has an estimated 350 returnees. Germany has documented about 250 returnees while Belgium puts its figure at about 130. Sweden has a total of 115 as the only other European Union nation with triple digits.

Most ex-jihadis who return to France are arrested and charged with terrorism. With justice system moving at a snail's pace, people who left in 2013 and returned quickly are only just going on trial next month, according to Xavier Nogueras, a lawyer who represents more than two dozen of them. He spoke with the AP in an interview before Friday's attacks.

Two police officers walk outside a flat in London February 26, 2015. Local media reported that the flat is the former home of Mohammed Emwazi, AKA 'Jihadi John,' a British citizen who went on to join Al-Nusra and then ISIS. REUTERS/Stefan Wermuth "The justice system is trying to make the effort to figure out who is dangerous and who is not, but because they don't have the manpower, they put them all in prison, and that can make them dangerous," he said. "There are so many of these people who now wait in prison, without knowing their fate. They are going to get more frustrated, ask 'why are you leaving me in prison to rot?'"

He estimates that two of his 25 cases are truly dangerous, and he said he has no interest in defending committed terrorists: "There's going to come a time when I'm going to have to stop this."

In Britain, 114 are awaiting trial while 21 have been convicted.

Petter Nesser, a senior researcher at the Norwegian Defence Research Establishment, said those who commit terror attacks in Europe are both former foreign fighters and others who are simply inspired by the radical rhetoric. But the deadliest attackers, he said, have a background in jihadi warfare.

Nesser said that there are many ways of tackling returnees, and one way "is to combine prosecution and preventive moves."

"Right now, we do not know what actions really help," he said.

(Casert reported from Brussels, and Jamey Keaten and Thomas Adamson in Paris contributed to this report.)