IBRAHIM ZAHAR, Tunisia » The same day that Jabeur Khachnaoui was gunning down foreign tourists at the National Bardo Museum in Tunis last month, one of his older brothers was on the other side of town, attending a rally against terrorism.

The two brothers grew up in a family with three other siblings in this small hamlet of olive groves in Tunisia’s impoverished southwest. They were close but in just a few years had ended up deeply divided in their outlooks, much like their country itself.

The older brother went to study philosophy in Tunis, while the youngest brother stayed at home and became intensely religious. After the older brother read about the attack online late that night, it was friends who broke the news to him that Jabeur Khachnaoui was one of the perpetrators.

"I felt very confused," he said in an interview in his home village. Still wrestling with the shame attached to his brother’s actions, he asked that his first name not be published. "I never thought he would do this. Terrorism is a strong hypnotizer, and it happened in the blink of an eye."

"We apologize to the whole world even though we are not responsible," he added.

Interviews with family members and neighbors of both gunmen whose March 18 shooting rampage killed 21 foreign tourists and a police officer in the national museum similarly revealed equal measures of shock and shame. Both gunmen were killed by anti-terrorist forces but not before they had committed the worst terrorist attack in this country in more than a decade.

The radicalization of the two gunmen, Jabeur Khachnaoui, 19, and Yassine Abidi, 26, is by now a familiar tale in Tunisia. But the scale and audacity of this latest attack have revealed not only how vulnerable Tunisia is to the radical Islamist groups proliferating in North Africa, but also how deeply split, like the Khachnaoui family, Tunisian society is growing as a whole.

Since its revolution, Tunisia has navigated a tense constitutional crisis centered on how to define the role of Islam in government and society, and it recently completed a successful democratic transition, alone among the nations swept up in the Arab Spring.

The government insists that it is steadily breaking up militant networks and rooting out sleeper cells. But the gunmen’s families blame radical preachers and online sources for brainwashing them, and yet others warn that young Tunisians are still being drawn into militant groups.

Even as the Tunisian government said it had killed or captured most of the group behind the museum attack, another group claimed responsibility and threatened more attacks.

Despite hundreds of arrests and raids against militant groups in Tunisia in the past two years — Prime Minister Habib Essid says his government has arrested 400 suspects since he took office Feb. 5 — jihadi networks are still active, luring and training recruits.

According to the authorities, the two gunmen and a third accomplice who procured the weapons for them traveled to neighboring Libya, which has descended into chaos since the Arab Spring. There, as recently as December, they trained for their mission to attack tourists at the National Bardo Museum, security officials have said.

Two other members of the group that helped plan the attack had returned from fighting in Syria, Interior Minister Najem Gharsalli said, touching on another concern — the estimated 3,000 men who have gone abroad to fight jihad and who may be a serious threat to the country on their return.

Khachnaoui became fanatically religious two years ago, according to his elder brother. The youngest son, he remained at home alone with his parents and grandfather and went to school by bus every day in the nearby town of Sbiba. It was there that he became drawn to radicalism. "All the trouble started in Sbiba," his brother said.

"He has talked in a radical way since 2013, he would fast and stay up all night reciting the Quran," his brother said. "He was always a strict person, and people would feel this awe around him. He would not make eye contact with women."

"My father felt very afraid when he saw how radical he started to be," he said. "But my brother assured us that however radical he was, he would never go to Syria."

On Dec. 8 his younger brother ran away from home. He called his father sometimes but always via Skype, and it was never clear where he was.

At one point he told his father that he was training to fight in Syria and would never return to Tunisia. Yet more recently he called to ask his father to help arrange a medical certificate because he was going to miss his baccalaureate exams — which were scheduled for two days after the Bardo attack.

The elder brother said he had tried to draw Khachnaoui toward philosophy, to broaden his view of the world. And he had thought he was making some headway, he said, clearly dismayed. Instead, Jabeur Khachnaoui’s father fetched his body home for burial last week.

"My brother was a victim of dark and extremist ideas," the elder brother said. "I don’t want to lighten his deeds, but he was much loved in the area."

The second gunman, Abidi, hid his militancy more carefully. He had also grown more religiously observant three years earlier, and attended morning prayers in a nearby mosque, but otherwise had showed little sign of extremism.

"He was not a radical," said his uncle, Mohamed Abidi, 63, a retired public servant. "He worked and he tried to make something of his life." He had started a new job and bought expensive new clothes just days before the attack, according to a cousin, Dalel, who asked to be identified only by her first name.

Yet Abidi was part of an underground cell originally formed by the Islamist movement Ansar al-Shariah, the police said after the museum attack.

Ansar al-Shariah was set up soon after the 2011 revolution by a Tunisian member of al-Qaida, a close disciple of Osama bin Laden, called Seifallah bin Hussein, also known as Abu Iyadh.

Its members took over mosques and began an aggressive campaign to recruit young followers with preaching and charity events. "You could find them everywhere," said a businessman in the neighborhood who grew up with Abidi. "They had tents all down the street."

When the government outlawed Ansar al-Shariah — accusing it of orchestrating an attack on the U.S. Embassy in 2012 and two high-profile political assassinations in 2013 — many members fled the country and others went underground.

Abidi hung out with two friends who lived in the same clutch of small streets in Upper Omrane, a working-class neighborhood in northern Tunis.

They frequented a radical mosque in the nearby neighborhood of Jebel Ahmar, and the group rented an old wedding hall for their prayer meetings, the businessman said. He asked not to be named because he did not want to show disrespect by talking about the family.

He said he was not surprised the jihadi groups still had cells in the city. "They started in the working-class neighborhoods," he said. "It was only when things became insecure for them that they went to the mountains."

In the southwest, where Khachnaoui lived, there is little economic opportunity. Families keep small herds of sheep and sparse olive groves. Most of the young people move to the cities to find work. Some go off to war.

Khachnaoui’s siblings said that 150 young men had gone to fight in Syria from the nearby town of Sbeitla, and at least 17 have died there so far.

Many blame the failure of successive governments to tackle the underlying causes of unemployment and poverty that fuel radicalization.

"The government has done nothing for regional development," said Alaya Allani, a professor of history at Manouba University in Tunis and an expert on Islamist movements. "There have been a lot of arrests from that area but nothing to improve the social base."

In the village the shame is heartfelt. "We are in so much pain for the tourists, we have been unable to eat," said Manoubi el-Masoudi, 60, who sells animal fodder in the village. "Such an action is alien to our society."

He spent 20 years working abroad as a younger man before setting up his own business, and he warned that the solution would not be swift. "We cannot cure this dilemma with bullets and killing and police but only with more culture and development."