The man who allegedly orchestrated France’s deadliest terrorist attack in half a century was introduced to radical Islam by a charismatic janitor who led a band of social misfits and petty criminals through military-style training exercises in a Paris park.

At the time, Chérif Kouachi was a pot-smoking pizza delivery man, drifting through life with only the vaguest of attachments to religion.

But his indoctrination more than a decade ago would prove fateful. It fired a zeal for violence that was apparently never extinguished despite an aborted trip to Iraq to battle U.S. forces, a three-year prison term and a long stretch when Kouachi convinced those around him — and perhaps even French law enforcement — that he had given up his dream of dying a martyr.

Now Kouachi and his brother are the subjects of one of the largest manhunts in French history, a day after allegedly gunning down a dozen people in an attack on a satirical newspaper in the heart of Paris.

“This was the realization of a long-term obsession,” said Myriam Benraad, a researcher at the Paris-based Sciences Po university who has closely studied the group of young men, Kouachi included, who were indoctrinated by the janitor and self-styled preacher at a northern Paris mosque.

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Kouachi’s long-standing involvement in France’s radical Islamist movement is bound to raise questions about why the 32-year-old Paris native was not being more closely tracked by French intelligence services.

U.S. officials confirmed Thursday that the brothers were on the no-fly list, which contains the names of U.S. citizens and residents as well as foreigners not permitted to fly into or out of the United States because of specific security concerns. There are about 47,000 names on the list, the vast majority of whom are foreigners.

Meanwhile, U.S. officials were carefully reviewing all available information about Kouachi to determine whether he could have used his low profile in recent years to quietly slip out of the country for terrorist training. U.S. officials said Thursday that his older brother, Said Kouachi, 34, appears to have traveled to Yemen in 2011 in an effort to link up with the al-Qaeda affiliate there.

Authorities are also looking into whether Chérif Kouachi made a more recent trip to Syria. So far, there’s no direct evidence that either brother had done so.

For more than a year, European security officials have been fixated on the danger posed by the thousands of young men and women who have left the continent to fight in Syria’s ever-deepening civil war. Many have returned home, and intelligence agencies have scrambled to track their movements amid calls by the Islamic State for Europe’s Muslims to bring the war to the West.

But Kouachi was part of an earlier generation of converts to radical Islam, one that predates the chaos unleashed following the Arab uprisings of 2011 and the declaration of a caliphate by Islamic State leaders in Iraq and Syria last year.

Like many of those seduced today, Kouachi was young and directionless when he fell under the spell of radical Islam. He worked odd jobs, smoked pot, drank and engaged in petty theft, said a lawyer who later represented him.

His guide to a different sort of life was Farid Benyettou, who, like Kouachi, was a young man of Algerian descent living in the hardscrabble neighborhoods of northern Paris’s 19th arrondissement.

But unlike Kouachi, Benyettou was able to mesmerize his peers with a seeming mastery of the Koran and intrigue them with his murky ties to North African militant groups. Despite his humble job washing floors, Benyettou became a direct challenge to the leadership of the local Addawa Mosque.

“He was a mix of a rock-and-roll star and a prophet,” Benraad said. “He was very cool, very well-spoken. And he was much more fun than the imam at the mosque. These guys would go to the mosque and be bored because the imam is speaking Arabic, and they don’t speak Arabic. They were convinced that the mosque wasn’t radical enough for them.”

Fueled by anger over the U.S.-led invasion of Iraq in 2003, Benyettou organized his followers into a cell of young men who took their name — the Buttes-Chaumont network — from the local park where they worked out and prepared for the day when they would do battle with American troops.

The men were surprisingly open about their extremist beliefs and their intentions, and the group became a prominent part of the radical Islamist scene in Paris.

“It wasn’t a secret. They weren’t ashamed of any of it,” said Amel Boubekeur, a French sociologist who interviewed several of the men, including Benyettou.

Using Benyettou’s militant connections, a number of his followers flew to Damascus and then traveled onward to the Iraqi war zone to link up with militants from the local al-Qaeda affiliate. Kouachi was prepared to join them.

“Farid told me that the scriptures offered proof of the goodness of suicide attacks,” he told documentary filmmakers from channel France 3 in 2004. “It is written in the scriptures that it’s good to die a martyr.”

But Kouachi didn’t get the chance. He was arrested by French police in 2005, just days before he planned to travel.

He served three years in prison before being convicted in 2008 of “criminal association in connection with a terrorist enterprise.” He was subsequently released for time served.

If Kouachi was behind Wednesday’s attacks, he is not the only former member of the Buttes-Chaumont cell who has gone on to commit violence with international reverberations. Another alumnus, French-Tunisian Islamist militant Boubaker el-Hakim, has been implicated in the assassination of two prominent Tunisian politicians in 2013 and has been linked to the Islamic State.

Kouachi’s older brother, Said, was not known to be a prominent member of Buttes-Chaumont. Jean Charles Brisard, a Paris-based terrorism and security expert, said Chérif probably recruited Said into radical Islam.

The public trail of Chérif Kouachi’s life went cold after his 2008 release, leading to speculation among analysts and officials that he may have been deliberately lying low to avoid detection.

If so, even the attorney who represented him after his 2005 arrest was fooled.

“He had been caught up in a religious movement. But I had the impression that he’d moved on,” said the attorney, Vincent Ollivier, during a brief interview at his Paris law offices on Thursday. “Apparently that wasn’t the case.”

Virgile Demoustier in Paris; Greg Miller, Adam Goldman and Terence McCoy in Washington; and Karla Adam in London contributed to this report.