PARIS >> Ten young Muslim men, bored by a mundane life in France and haunted by a “feeling of uselessness,” as one put it, were seduced by a leading Islamic State recruiter in Europe in 2013. Within months, they were in Syria under the watchful eyes of hooded, Kalashnikov-wielding militants, doing push-ups, fiddling with weapons and imbibing the ideology.

But the harsh regimen, most have since told investigators, was not to their liking, and it was not long before they hastened back to their families in the Strasbourg area, where they were almost immediately picked up by French authorities.

What to do with hundreds, perhaps thousands, of such young men in Europe is now among the biggest challenges facing governments and security services.

After the Paris and Brussels terrorist attacks, which were carried out in part by Europeans who had spent time in Syria with the Islamic State, France and other countries are grappling with how far to go in tightening laws to prosecute, monitor and restrict the movements of returnees.

At the heart of the debate is whether to take pre-emptive legal action against people who have not committed terrorist acts or even been implicated in a plot, but who have simply been to Syria and possibly received training in Islamic State camps.

Several urgent factors have propelled the debate: the steeper risks of terrorist attacks, the fact that monitoring the sheer numbers who have returned is overwhelming security services, and the difficulty of building cases against suspects who may have been trained and indoctrinated in distant lands.

At least 14 European countries have made receiving terrorism training a criminal offense. Nine have made travel from the war zones of Syria and Iraq an offense.

In France, more than 900 people have left the country to be jihadis, making it Europe’s biggest wellspring of foreign fighters. The government is now debating whether to allow house arrest even if there are merely “serious reasons for thinking” someone has been in an overseas war zone.

Two trials involving French jihadi cells were held in Paris this month, with the defendants in one of them receiving sentences of up to 10 years on charges of “criminal conspiracy with the aim of preparing acts of terrorism.”

Setting policy for dealing with returning jihadis is just one example of how Europe, like the United States after the Sept. 11 attacks, is being forced to weigh security concerns against civil liberties.

Even before the Paris and Brussels assaults, cases like the one in Strasbourg were highlighting the difficult choices posed by European citizens coming home from Syria.

At least 1,300 European jihadis have returned to the Continent, and those are only the ones identified by the police. Three times as many Europeans may have gone to Syria, some slipping back undetected, with disastrous consequences.

One member of the Strasbourg group, Foued Mohamed-Aggad, returned after the others and was not detected by the authorities. He ended up as one of the three killers at the Bataclan concert hall in Paris, where 90 people were shot dead Nov. 13.

Of the nine others from Strasbourg known to have gone to Syria, two were killed there in January 2014, but seven returned, were quickly arrested in May 2014 and now await trial.

The record of their interrogations afterward is a dance of denial that they intended any harm in Europe.

“This is a radical, extremist Islam which has nothing to do with basic Islam, and which I disavow,” Foued’s brother, Karim, told investigators, according to documents recounting the interrogations.

There is no policy covering all of Europe, and in some instances the returning jihadis are not immediately put in jail. Though they are almost always placed under some form of monitoring, this puts a big strain on the police and intelligence services.

Fifteen to 20 officers per jihadi are required for constant surveillance, said Magnus Ranstorp of the National Defense College in Sweden, a country where about 80 have returned. In Belgium, the biggestper capita furnisher of jihadis in the European Union, 50 to 120 have returned.

But after the recent attacks, and with the Islamic State promising more terrorist strikes, a period of relative laxity is over, analysts said.

“They will be arrested, and the evidence will be gathered,” said Edwin Bakker, director of the Center for Terrorism and Counterterrorism at Leiden University in the Netherlands.

More prosecutions are being undertaken preventively, even when there is little firm evidence of a plot, as the French examples show.

But the Strasbourg cases illustrate the difficulties of building a prosecution.

The Strasbourg group, all men in their mid to late 20s, had met in hookah bars and fast-food joints. They included a delivery driver, a service station attendant, a physical education instructor, several who were unemployed and a petty criminal.

Their departure for Syria in December 2013 had been well publicized. So when they returned, from February to April 2014, France’s internal security agency, the DGSI, knew whom to look for, and where.

After their return, the investigators listened and watched, finally pouncing on May 13, 2014, in the same working-class neighborhood they had left.

In the eyes of authorities, their crime was simple: being enlisted by a well-known French recruiter, traveling to Syria and Islamic State safe houses or training centers, training with Kalashnikov rifles and other weapons, and returning to France.

Yet in their interrogations, none of the men admitted wrongdoing, and several insisted that they had gone to Syria for humanitarian reasons.

The men now claim to reject Islamic State ideology and tactics and profess regret for their “stupidities,” as one put it under questioning.

But investigators are well aware that Islamic State training manuals urge recruits to practice the art of taquiya, or concealment. One suspect in Strasbourg, Mohamed Hattay, went through five interrogation sessions before admitting he had even been to Syria.

As a result, authorities try to find evidence from mobile phones, social media posts and other sources that shed light on motivations and true intentions.

“We know sometimes people try to suggest they are disillusioned, but they are just hiding it,” said Gilles de Kerchove, the EU’s counterterrorism coordinator.

“When we intercept their communication, we discover it is not true,” he said.

The seven Strasbourg suspects portray themselves as full of regret for having made the trip to Syria.

“I wasn’t brought up with that way of thinking, and I am totally against it,” one of the men, Miloud Maalmi, told interrogators in 2014.

“When I was there, they kept me isolated, and they made me undergo truly abominable things,” Maalmi said. “I want to turn the page.”

Another of them, Radouane Taher, said, “Once I got there, I understood very quickly what a mess I had gotten myself into, and that I had fallen in with a bunch of madmen.”

Even escaping was a risk, Karim Mohamed-Aggad said. “Each one of us, we had to play games in order to flee the country, because our lives were in danger,” he said.

Yet investigators said they had found photographs on Facebook, phones and computers showing some of the men posing with the Islamic State flag, wearing military gear and holding rifles. There were also photographs of decapitated corpses, the investigators said.

In addition, all had been recruited in France by Mourad Fares, who said in a 2014 interview with Vice that he was the “principal recruiter” for jihadis in France.

He met Maalmi at a McDonald’s in Kehl, Germany, across the river from Strasbourg.

“All the jihadists the newspapers are talking about, they passed through me,” Farès told Vice from Syria.

Fares is now in jail in France.

It is difficult to tell from the mostly bland answers in the transcripts how radicalized the suspects became, though the investigators were able to catch them in inconsistencies and evasions.

“This interrogation contradicts the two preceding it; there being no place in this part of the story for ‘humanitarianism,’” the investigators noted in their summary of statements by Ali Hattay, another suspect.

Liberal-minded French jurists have criticized what they call the “criminalizing of intentions.” But France remains under a state of emergency after the Nov. 13 attacks.

“The people that return now, they will probably be hunted down,” said Bakker, of the Center for Terrorism and Counterterrorism.

“The general approach is really to try to convict these people,” he added. “In the past there was the attitude, ‘Let it go.’ But we’ve really gone beyond that.”

© 2016 The New York Times Company