Already there are signs that some of the 5,500 Tunisians who have gone abroad, according to United Nations estimates, are seeking new targets at home and in Europe, where Tunisians have been implicated in several recent terrorist cases in France and Germany. For Tunisia, there is no easy solution. The new Constitution does not allow the government to bar them. They can be locked up for joining a terrorist group, or for committing crimes abroad, but cases are hard to build and charges difficult to prove. The president proposed amnesty, only to be vigorously opposed. So the country has settled into a harsh, and potentially illegal, system of monitoring. Domestic opponents and international rights groups, including Human Rights Watch last year and Amnesty International in a report issued in February, are protesting it as counterproductive. The threat of imprisonment and torture is deterring many from returning home, in effect rendering them stateless. Some are hiding in Turkey and Europe, where they may be ticking time bombs. There is no government program to de-radicalize returning fighters or reintegrate them into society, said Ridha Raddaoui, a lawyer and co-author of a new report on terrorism in Tunisia by the Tunisian Forum for Economic and Social Rights. Families of suspects and fighters who have returned are persecuted rather than supported, he said. “The methods are pushing people to terrorism,” Mr. Raddaoui said. “We think the victims of terrorism are also the families.” Tunisian fighters still hold prominent positions in the Islamic State and the Nusra Front in Syria. But of greater threat to Tunisia are those in Libya affiliated with Al Qaeda in the Islamic Maghreb, which is active in a half-dozen countries across North Africa and vows to bring Islamic rule to Tunisia and all of North Africa. “Their aim is still to destabilize the state,” said Col. Mokhtar Ben Nasr, who heads military analysis at the Tunisian Center for Global Security Studies. “They want to make people rise up over poverty and injustice, and they do propaganda to that effect.” The violence the returning fighters and others who have been radicalized can sow has already been witnessed in Tunisia, which has suffered a drastic rise in instability and terrorism since the Arab Spring uprising of 2010-11. After the revolution, Qaeda followers, released from prison or returned from exile, were quick to exploit the new freedoms, taking over the mosques and establishing a network of cells across the country and training camps in Libya. While many interpreted the Arab Spring as proof that people power was more potent that terrorism, Al Qaeda was faster to organize than any other political movement and became the main terrorist threat to the country, according to the report on terrorism. “The terrorist current was one of the main beneficiaries of the Tunisian revolution,” the report concludes. “From the first months it quickly profited from the release of its detained members and from the climate of freedom to rebuild itself.” Terrorists benefited from outside support, but their growth was critically linked to Tunisia’s internal problems, the report also said. By 2013, insurgents were embedded in the mountains on the western border with Algeria, and groups in the cities began a campaign of political assassinations, ambushes on the police and army, and suicide attacks. Since then, Tunisia has been hit by more than 50 terrorist attacks, including increasingly spectacular assaults orchestrated by groups based in Libya. More than 70 people were killed in mass shootings in 2015, many of them foreign tourists at the national Bardo Museum in Tunis and at a beach resort in Sousse. A suicide attacker killed 12 members of the presidential guard in central Tunis in November the same year. In their most ambitious attack to date, hundreds of Tunisian fighters tried to seize control of the Tunisian border town of Ben Gardane in March last year and fought Tunisian security forces in the streets for several hours. Tunisian officials insist their security forces are getting a grip on terrorism inside the country. Indeed, the rate of attacks has fallen off in recent months. “There has been a lot of work by the police and army,” Colonel Ben Nasr said. “They have dismantled the logistics and recruitment networks, and there is no longer this support system that there was in 2012 and 2013.” United States airstrikes in Libya last February targeted a major Tunisian camp, where the perpetrators of the 2015 attacks had trained, and helped push the Islamic State out of the city of Surt — both major blows to the jihadists in Libya. Up to 400 Tunisian fighters were killed in Libya in the past year, according to Libyan security forces, yet hundreds remain at large, dispersed around the country and in camps in southern Libya. Al Qaeda’s leader, Ayman al-Zawahri, called on Tunisians in a recent audio message to wage jihad to “excise tyranny” from the country. Researchers studying radicalization in Tunisia say police harassment is still pushing young people to run off and join the extremists. “Their territory is diminishing but not their numbers,” said Mohamed Iqbal Ben Rejeb, who founded the Rescue Association of Tunisians Trapped Abroad after his brother was recruited to fight in Syria. In December, President Béji Caïd Essebsi of Tunisia floated the idea of a pardon for returning fighters, saying there was no more room in the prisons. But after opposition protesters flooded the news media and television chat shows, he ruled out the idea, promising there would be no pardon and no amnesty. Islamist leaders, who have been sympathetic to young jihadists, have joined him in insisting that all returning fighters go before the courts on their return. “Those who survive will have to go before justice and pay for what they did,” Abdelfattah Mourou, a co-founder of the Ennahda party and deputy speaker of Parliament, said in an interview. “They killed people, and they have to pay.” But human rights activists and others say that leaving the jihadists no way out allows the cycle of radicalization to continue. Mr. Ben Rejeb said he knew of dozens of Tunisians who wanted to come home, many of them trapped in jails in Syria and Libya. The government will not be able to stop fighters from returning, said Mr. Raddaoui, the lawyer, and it may well find it difficult to pin crimes on them for lack of evidence. “There are no autopsies, no police reports,” he said of the crimes committed on distant battlefields. Of greater concern, Mr. Raddaoui said, is that in reading the court papers of the 500 Tunisians who have been processed since their return, he has seen no remorse.