In all, about 12,000 Europeans left to fight with the Islamic State group and al-Qaida; about a third of those are now believed to be home, mostly living freely. Some are awaiting trial, but most never faced serious charges due to insufficient evidence.
And many more saw their travel plans thwarted, left behind to stew. How much of a threat do these avowed extremists living throughout Europe pose, and how equipped are authorities to handle them? The response has been, at best, improvised.
The impending releases of jihadi veterans could be considered "a fourth wave of returnees," according to Rik Coolsaet, a scholar at Belgium's Egmont Institute. "There are a number of personal frustrations and motivations that have pushed the kids in their journey to ISIS that we now have to address," Coolsaet said. "If we don't address it now, the environment will remain as conducive for this kind of jihadi violence."
Farid Benyettou, an ex-jihadi who served four years in prison and has now publicly renounced extremist violence, fears Europe is not ready. Once nicknamed the "imam Voltaire" after the school he left to become a backroom preacher to young Muslims, Benyettou has written a book detailing his descent into Islamic extremism.
Among the members of the group he once led were Cherif and Said Kouachi, who gunned down 12 people at the offices of the satirical newspaper Charlie Hebdo in 2015. The cell epitomizes Europe's urgent question: Are the terrorism convicts verging on freedom like Benyettou, the Kouachis or somewhere between?
"These guys who are convicted today or who are awaiting trial will get out one day," Benyettou told The Associated Press. "And that's the issue, in fact: What kind of preparations will there be for their release?"
Terrorism prison sentences in Europe until very recently averaged about six years, compared to 13 years in the United States, according to Europol data. Since 2015, sentences have risen, but remain well below U.S. levels.
"The danger is the risk of recidivism. We should not be too quick to believe certain terrorists who say they are repentant," Catherine Champrenault, the Paris prosecutor general, recently told Le Monde newspaper.
France, which has been struck repeatedly by Islamic State fighters and sympathizers, will free 57 inmates — about half its current population of terrorism convicts. In Britain, where nearly half of terrorism sentences are four years or less, 25 inmates are due for release — fully three-quarters of those convicted under one of the country's main terrorism statute as of mid-2017. In Belgium, 80 acknowledged foreign fighters already are free and as many as 44 others will be joining them. In Spain, 21 of 34 returning extremists already were free as of late last year. And in Bosnia and Kosovo, every jailed foreign fighter will go free.
In just those countries alone, the total runs to more than 200, according to the AP's count. And the actual number is undoubtedly higher because not every country releases data — notably Germany, which had nearly 1,000 residents leave for the war zone but releases no comprehensive figures on convictions or releases.
The most recent attack blamed on returning foreign fighters was in March 2016, when an Islamic State cell of jihadis set off suicide bombs at the Brussels airport and in the metro. The network already had attacked a Paris-to-Brussels high-speed train in August 2015 and orchestrated a brutal assault on Paris in November 2015. In all, the cell killed 162 people.
Still, the overwhelming majority of returning jihadis have not been arrested and have caused no harm. Many of the homegrown extremists are young and poor with limited education, and feel cut off from society.
Once free from prison, no programs or policies govern them. France applies a range of post-release constraints, ranging from requiring those freed to periodically check in with authorities — as is the case for Benyettou — to home detention.
In Bosnia, where all 23 terrorism convicts are leaving prison, the Justice Ministry said local housing, social workers and employment agencies are notified, but that there's little capacity to do much more.
Britain has limited counseling and monitoring, and offers a voluntary re-integration program. And Spain began "re-education and reinsertion" in 2016 in which terrorism inmates are evaluated for risk and undergo counseling. But the government said only 10 out 146 extremist inmates have undergone the much-hyped program.
For Benyettou, getting support as he planned a future was crucial to abandoning extremism. He studied first to be a nurse only to see that dream evaporate because of the Charlie Hebdo attacks. He is now training as a trucker.
"Everyone has setbacks in life, plans that don't happen. For me, it's my past," he said. "You bounce back and try to do something besides to fall back on this logic of victimhood." And that's where many fear Europe will fail the next wave of terrorism convicts
"Sending them back to exactly the same circumstances that caused them to take up violent extremism," said Richard Barrett of the Soufan Group, an expert on violent extremism. "Well, unfortunately, you're probably going to get the same result."
Aritz Parra in Madrid; Jill Lawless in London, and Sabina Niksic in Sarajevo contributed.