The 10-year evolution from easily spooked amateur to hardened killer is a story of steadily deepening radicalism that occurred virtually under the noses of French authorities, who twice had Chérif in their grasp. After the arrest of Chérif in 2005, when he was no more than a fledgling jihadist, he spent 20 months in prison. There, he met and became an acolyte of Al Qaeda’s top operative in France, Djamel Beghal, who had been dispatched to Paris to set up a cell aimed at attacking United States interests here, French counterterrorism officials said.

He also befriended a convicted robber, Amedy Coulibaly, who would later synchronize his own terror attack with the Kouachi brothers, killing a police officer and staging a siege inside a kosher supermarket in the days after the Charlie Hebdo carnage, bringing the death toll to 17.

Much remains unclear about their lives. But thousands of pages of legal documents obtained by The New York Times, including minutes of interrogations, summaries of phone taps, intercepted jailhouse letters and a catalog of images and religious texts found on the laptops of Chérif Kouachi and Mr. Coulibaly, reveal an arc of radicalization that saw them become steadily more professional and more discreet.

They shaved regularly, eschewing the conspicuous beards worn by many Islamists. They dressed in jeans and basketball sneakers, offering no outward hint of their plans or jihadist beliefs.