We Now Know How The Charlie Hebdo Attackers Radicalized

French policeHandout photos released by French Police in Paris of Cherif Kouachi (L), 32, and his brother Said Kouachi, 34.

One of the Charlie Hebdo attackers who led a deadly shooting rampage at the magazine's Paris office last week was radicalized by a militant jihadist network that has produced some of Europe's most dangerous extremists, the New York Times reported Monday. 

Chérif Kouachi reportedly trained with the Buttes-Chaumont group, a homegrown cadre of radicalized young Muslim men from an immigrant neighborhood in the 19th Arrondissement whose members were arrested in 2005 for sending some of their own to fight in Iraq. 

While in jail, Chérif Kouachi was recruited by the French-Algerian jihadist Djamel Beghal who had plotted to bomb the United States Embassy in Paris in 2001. It was during this time that Beghal recruited Amedy Coulibaly, the shooter who killed four hostages at a kosher market in Paris on Friday.

Buttes-Chaumont may have largely disbanded when the 2005 arrests were made. But its members' most important connections were made with other extremists in French prisons. The boys of Buttes-Chaumont, intermingling with prominent jihadists in jail, became increasingly hardened and committed to radical Islam.

"He [Cherif] was much more radical when he was judged in 2008 than he was in 2005, when he was arrested," Buttes-Chaumont defense lawyer Dominique Many told the Times. "So perhaps in jail he became what he is today, the Kouachi that we knew these last days."

REUTERS/Stoyan Nenov A man holds a placard reading "I am Charlie" to pay tribute to the victims following a shooting by gunmen at the offices of weekly newspaper Charlie Hebdo in Paris, in front of the French embassy in Sofia, January 9, 2015.

According to a leaked French intelligence report that the Times cites, close to 300 Muslim inmates were deemed radical "time bombs" who officials believed would pose a threat once they were released. 

The men of Buttes-Chaumont comprise just a handful of the several hundreds of young, aliented Muslims in France who have traveled to fight with jihadists in Syria and Iraq. French officials are now starting to question whether they may have underestimated the influence of extremist networks like Buttes-Chaumont.

One of the group's former leaders, Tunisian national Boubaker al-Hakim, has been actively building a network of fighters across Northern Africa and in European immigrant communities in the name of the Islamic State. 

While it is unclear whether or not Cherif's older brother, Saïd Kouachi, was also a member of Buttes-Chaumont, authorities have confirmed that he and his brother received training from an Al Qaeda branch in Yemen.

It was in Yemen where Said befriended Umar Farouk Abdulmutallab, the infamous "underwear bomber" who tried to bring down a US-bound plane back in 2009, according to the Wall Street Journal. The two were reportedly neighbors while attending the same Arabic school in San'a.