It was a decade ago, the war on terror was in full swing, Iraq was on fire, and every day there were images of American combat soldiers on patrols on French TV.
In their 20s, one of the brothers' favorite spots was the city's fifth largest public park, the Buttes-Chaumont, with open meadows, ponds, stunning overlooks onto the city and curiously close to the place where for centuries criminals were hanged.
The brothers went to the park for sports training and exercise, but it is there in the Buttes-Chaumont where they also met with more than a dozen other similar young men, who would all eventually be known to police for a terrorism plot to fight as jihadis in Iraq. One of the Kouachi brothers would be jailed for his involvement in that plot.
But the brothers' journey would go far beyond the group of men plotting in the Buttes-Chaumont a decade ago.
Theirs would be a journey from youths training in a park to becoming some of the worst terrorists of the modern world.
They would go from meeting disenfranchised youths to rubbing shoulders with influential, international jihadi leaders.
And eventually the unknown brothers' actions, along with their compatriots', would culminate in some of the most bloody and horrific crime scenes ever known in Paris, plunging the City of Light into darkness and despair.
Now, they are dead, killed by police in a gunfight after they attacked the offices
of Charlie Hebdo
, a French magazine with a history of mocking Mohammed
. Twelve people were killed, including several top editors. The brothers said they were avenging the prophet.
The Kouachi brothers' story has been put together from numerous sets of French court documents obtained by CNN in recent days, along with interviews with numerous terrorist experts and former French government terrorism officials.
The Kouachi brothers, of Algerian descent, lost their father when they were teens, according to documents and interviews. Their mother, unable to care for them, eventually sent them off to a state school for children with special needs, in the countryside. She, too, would die before too long. Their schooling would end before university, and they eventually washed up in Paris' 19th arrondisement.
They survived as best they could, with only odd jobs. Cherif, the younger brother, worked as a pizza deliverer, according to documents.
Among those the brothers met were Farid Benyettou, an influential and leading figure who mentored many of the young and disenfranchised youths at the local mosque Adda'wa, known as the "mosque de Stalingrad," according to one set of documents obtained in conjunction with L'Express, a French news magazine.
Neither brother is believed to have been particularly religious. But Cherif, according to documents, was especially drawn by Benyettou, first to the idea of religious zeal, then to Islamic radicalism, and finally to the larger idea of jihad and fighting as a jihadi, the documents show. Many of the members of the gang would similarly credit Benyettou.
Benyettou would serve as a common link for the gang of the Buttes-Chaumont, as the group would come to be known to police in 2004 and thereafter. The group worked and plotted together to send young Islamic immigrants from France to Iraq to fight jihad against Americans there, police said.
Three of the men in the gang had already traveled to Falluja in 2004, when it was under the control of al Qaeda in Iraq's leader, Abu Musab al-Zarqawi. Cherif said he, too, wanted to travel to Iraq, going through Syria, "to go and combat the Americans," according to documents.
Benyettou talked to the youths about discovering their identities, and finding their "roots." The roots he referred to were those of religious zealots, and eventually jihadi fighters. He became, for them, a sort of religious leader, a trainer of the Quran, and how it applied to being a jihadi. And Benyettou talked to the young men about abuses committed by the United States in Iraq, most notably what happened at the Abu Ghraib prison.
In the 2007 French court documents, Cherif Kouachi stated in a deposition
, "I was ready to go and die in battle," and "I got this idea when I saw the injustices shown by television on what was going on over there. I am speaking about the torture that the Americans have inflicted on the Iraqis."
The court documents, dated December 2007, stem from a 2005 arrest.
In the documents, prosecutors outlined strong details of Kouachi's interest in jihad, interest in martyrdom and strong links to anti-Semitism, attacking and killing Jews.
Kouachi said he came to the idea of jihad through Benyettou. Kouachi said he likely would not have pursued the idea had it not been for Benyettou.
Through Benyettou, Kouachi was studying how to wield arms and use Kalashnikovs. Under a section titled "Motivations of Influence" describing Kouachi, court records said he stated "the wise leaders in Islam told him and his friends that if they die as martyrs in jihad they would go to heaven."
Benyettou told them that "martyrs would be greeted by more than 60 virgins in a big palace in heaven."
The documents also said Kouachi had come to feel that "any place on Earth where there is such an injustice is justification for jihad; what was going on in Iraq was in his eyes such an injustice."
Kouachi said he didn't consider himself a good enough Muslim, and said he had only been to the mosque two or three times before he met Benyettou, and he had been smoking cannabis.
Kouachi told investigators he committed himself to the idea of jihad during Ramadan in 2004. He told his friends he was going to Syria to fight.
When police interviewed his accomplices they stated that Kouachi "said he was ready to firebomb and to destroy Jewish shops in Paris."
But when officials confronted Kouachi with that information, he told them "that's not exactly what I said. ... I don't hide having proposed anti-Semitic ideas, but I would note that I never really would have done that."
Kouachi, however, never made it to Iraq. He was detained, along with six others, including Benyettou, in 2005. They were charged with conspiring to commit acts of terrorism.
Kouachi told the court he was relieved he was detained. French journalist Magali Serre interviewed Kouachi's social worker, who said Kouachi said he was finished with so-called jihad, documents show.
But in prison the real radicalization began for Cherif Kouachi, and it would ultimately lead him and his older brother, Said, to terrible acts.
Cherif Kouachi was "in prison with other hard-liners, including a central figure in the al Qaeda networks in Europe," said Jean-Charles Brisard, the head of the French Center for the Analysis of Terrorism.
The central figure in prison was Djamel Beghal, also of Algerian origin, who was jailed for plotting to attack the U.S. Embassy in Paris in 2001.
"In prison, Beghal became the mentor, the spiritual mentor, and continued the work in some way initiated by Benyettou," said Brisard.
Beghal would prove to be pivotal to the evolution and radicalization of the brothers and their friends. It was only the latest step in Beghal's long history of mentoring future terrorists.
Once known as al Qaeda's premiere European recruiter, Beghal was convicted himself of conspiring to attack the U.S. Embassy in Paris. "Beghal was in direct contact with the highest ranking members of al Qaeda at the time," said Brisard.
Beghal's ties to al Qaeda stretch back before the 9/11 attacks in the United States against the World Trade Center and the Pentagon, sources told CNN.
He was put in charge of the plot to blow up the U.S. Embassy in Paris, according to court documents. He was ultimately convicted in a French court.
Beghal's mentor was Finsbury Park mosque cleric Abu Hamza al-Masri, who was only last week convicted on terrorism charges by a federal judge in New York. He was sentenced to life in prison.
Al-Masri is believed to have radicalized and influenced convicted terrorists shoe-bomber Richard Reid
and 9/11's Zacarias Moussaoui
Documents show Beghal later came to be referred to as "Abu Hamza," out of respect for Beghal's mentor.
Cherif and Beghal grew close during Cherif's time in prison, from 2005 through 2008, and it is believed the far more radical and jihadi-focused Beghal had a significant affect.
Another disaffected young and angry Muslim, also in prison, was Amedy Coulibaly
, the third attacker in Paris last week, who is accused of killing a French police officer and killing four more people at a kosher grocery store.
Coulibaly and Cherif Kouachi met in prison, where Coulibaly was serving a sentence for armed robbery, according to court documents.
The radical jihadi leader Beghal became a tie between Kouachi and Coulibaly.
Beghal got out of jail in mid-2009, and from that point on Kouachi and Coulibaly went to visit Beghal at his place at Murat, in Cantal. They would bring Beghal food and money.
Also part of the visits was Coulibaly's partner, Hayat Boumeddiene
, who is on the run and wanted by police as a suspect in the kosher grocery store incident.
Authorities say Beghal, Kouachi and Coulibaly in 2010 plotted to break out another imprisoned Algerian terrorist, Smain Ait Ali Belkacem. Belkacem had been imprisoned for 1995 bombings in the Paris Metro system.
Court documents detail how Kouachi and Coulibaly worked with Beghal, hiding their conversations by using code names over disposable cell phones.
In April 2010, both Kouachi and Coulibaly were even staying with Beghal, sometimes for days.
Sometimes Hayat Boumeddiene (called Coulibaly's female companion in documents) joined the visits.
Coulibaly is described in the documents as the logistics expert who was in charge of gathering the weapons and arms in the plot to spring Belkacem. There were elaborate details -- sophisticated computers, encryption, details of one-way tickets and handwritten notes in Arabic that when translated turned out to be recipes for poison.
In May 2010 in the Department of Seine Saint Denis, Coulibaly "was found to have illegally stored a huge cache of high caliber arms," including some 240 cartridges for high-powered machine guns "with the specific goal of seriously hurting people through intimidation or terror acts, in violation of numerous national security laws.
Kouachi, Coulibaly and Beghal eventually would be arrested for the plot. But while Coulibaly went to jail, Kouachi did not, because there was not enough evidence.
Coulibaly was released early, sometime in 2014. Beghal was sentenced to 12 years in prison and remains behind bars.