Finally, on Oct. 30, Ms. Saadi came home to find the police surrounding her house. A suicide bomber had blown himself up in Sousse, a seaside resort about an hour’s drive south. Another was caught before he could detonate his payload. The police confiscated the family’s computers and phones, and her husband spent the rest of the day at the police station. He called her later from there. “Yes, it is your child,” he told her. Aymen is now in prison.

In the weeks since the attack, Aymen’s trajectory from promising student to potential suicide bomber has shaken Tunisia, where the Islamist government has recently shown moderation by striking a compromise with its secular opponents. Homegrown suicide attacks, previously unheard-of here, are the latest sign of spreading radicalization among young people in a country that has become fertile ground for terrorist groups recruiting militants for the conflicts convulsing the region.

For now, extremist violence in Tunisia is on a low boil, with two political assassinations and 30 members of the security forces killed this year. But there is growing concern that hundreds of young volunteers have been recruited through a widening network of hardline Salafist centers and then trained to fight in Syria, with the potential to return home to cause more trouble, as Aymen and his companion did.

“Even if just 200 come back, that could cause real problems,” said Mehdi Taje, the director of Global Prospect Intelligence and a specialist on North Africa. And Syria is not the only place radicalized Tunisians have gone to fight. They have also been found with terrorist groups in Algeria, Iraq, Libya and Mali.

There is a precedent next door. Algerians who went to Afghanistan in the 1980s to fight Soviet forces returned to fuel an extremist movement and then a civil war at home that killed about 200,000 in the 1990s. Twenty years later Algeria is still dealing with insurgents, who have retreated into the desert.

The Tunisian police and army officials have warned of signs that the insurgents may be laying the groundwork for an armed insurgency in their own country, which lies between Algeria and lawless Libya.

Since Tunisia began the Arab Spring almost three years ago by ousting its longtime dictator, Zine el-Abidine Ben Ali, who had forcibly secularized the country, fundamentalist Salafist groups have sprouted in almost every town.

They draw thousands of young men and women to their centers, where they recruit volunteers ostensibly for missionary work in Tunisia, but also for jihad.

Some began vigilante attacks, including an assault in September 2012 on the United States Embassy in Tunis, days after the fatal attack on the American Mission in Benghazi, Libya. In the spring, armed groups also appeared in the hills bordering Algeria, apparently remnants of insurgents retreating from the French intervention in Mali.

Tunisia’s top general, Rachid Ammar, warned in a television interview in June that the militants lodged in the mountains on Tunisia’s western border were training quasi-military units and were set on overthrowing the Tunisian state.

“This is not terrorism, it’s a rebellion,” warned the general, who has since retired. “This is one of the stages of rebellion.”

The surge in youthful Salafist followers like Aymen was also evidence of a popular social movement. The Salafist centers provide open spaces for inquiring youth who are lured by charismatic preachers offering a stirring mix of camaraderie and talk of holy war and self-sacrifice in the name of God.

Although most Salafists insist their activity is focused on educational and humanitarian causes, it is clear from dozens of interviews with families of the recruits, analysts and government officials that a growing number of young men are being drawn into militancy.

In August, the government outlawed one Salafist group, Ansar al-Shariah, as a terrorist threat. Led by men who fought in Iraq and Afghanistan — many of them released from prison since the start of the Arab Spring — the group has recruited criminals and smugglers, and it has also drawn many young recruits into its ranks, security officials say.

The police say that since May they have smashed several cells of the group, detained 300 people suspected of being members, and killed or arrested its main military leaders. In October, six friends who were members were killed in a clash with security forces south of Tunis.

“We hit them very hard,” Mohammed Ali Aroui, an Interior Ministry spokesman, said in an interview. “Ansar al-Shariah is in the past now, you don’t see their signs or slogans or tents anywhere.” The group’s leader, Seifallah Ben Hassine, known as Abu Ayadh, is thought to have escaped to Libya, he said.

Under pressure, Ansar al-Shariah members have retreated underground, but recruiting for war continues, by Ansar al-Shariah and other groups, through religious associations. The volunteers are organized by a network of facilitators who supply money, cars and safe houses in various towns.

The recruits travel mostly through Libya, where they receive military training in camps in and around Benghazi. From there they fly to Turkey, which is the main access point for rebels entering Syria. The Tunisian police have set up border controls to stop anyone under 35 suspected of traveling for war. But Tunisia’s borders remain porous, as fighters and weapons cross to and from Libya.

Ms. Saadi says her son seems to have taken that route. Aymen, now 17, first tried to go to Syria in March when he left home, she said, but the police stopped him at one border check. In August, he took off again and was gone for over two months.

Aymen’s plan was always to go fight in Syria, but in Libya some insurgents ordered the two bombers to go back to Tunisia, his mother said. His mother learned of her son’s account from one of his prison wardens.

The Libyan insurgents, whose faces were masked, told the two youths that it was not the time to go to Syria. “The fight is in Tunisia right now, we want to create an emirate there,” they said. When the two youths showed some reluctance, the masked men insisted, saying they had no one else for the job. Government officials confirmed the account.

Aymen’s parents, a teacher at a primary school and an agricultural engineer, had hoped he would follow his older brother to university. He was good at math.

None of his school friends wanted to go to Syria, she added. She blamed other strangers, “older men and sheikhs with cars,” for influencing her son.

“It is a shame to call a young man a terrorist,” she said of her son. “I see that he is a victim, and the real big terrorists are still roaming on the streets and young men like my son can be influenced.”

BA/BA