Omar Ismail Mostefai was known to police as nothing more than a petty criminal before he became the first gunman identified from last Friday's attacks in Paris, which left at least 129 dead.
Identified by his severed finger, which was found in the rubble of the Bataclan concert hall, the 29-year-old was one of three attackers who killed 89 people in the bloodiest scene of the carnage. Two of the men, including Mostefai, blew themselves up, while one was shot by police.
Suicide bombers and gunmen linked to the Islamic State in Iraq and Syria (ISIS) militant group "were behind how the terrorist acts unfolded", Paris prosecutor Francois Molins said at a televised press conference late last Saturday.
He told reporters that three militant cells acting in unison staged the coordinated hits at the concert hall, restaurants and bars, and a football stadium.
Seven gunmen, all wearing suicide vests packed with explosives, died in the multiple assaults.
Mostefai was born in the modest Paris suburb of Courcouronnes and was one of four brothers and two sisters of Algerian descent. While he had eight petty crimes on record, he never spent time in jail. After he travelled to Syria last year, security services kept him on file for Islamist radicalisation.
"He was never before involved in any investigation into organised crime or a terrorist organisation," Mr Molins said.
Yet, at about half past midnight last Friday, he ignited a vest filled with explosives to kill himself when special security services stormed the music hall.
He and his two accomplices had entered the hall during a concert of the Californian hard rock band Eagles of Death Metal, killing several dozen defenceless people with automatic Kalashnikov rifles.
"We heard explosions, cries, I turned around and saw several people who were aiming at us and shooting randomly into the audience. They did not wear masks. The one I saw seemed very young, in his 20s," said Mr Julien Pearce, a journalist who was in the hall.
Police yesterday detained six people close to Mostefai, including his father, brother and sister-in-law, said judicial and police sources.
Mostefai's brother said he could not believe his sibling was involved. "It is madness... I was in Paris yesterday and I saw the mess," he told Agence France-Presse.
Police have identified two cars used in the attacks. One was a black Seat that was found abandoned in the suburb of Montreuil yesterday. Several Kalashnikov rifles of the sort used in the attacks were in the car, a judicial source said.
Witnesses said the car was used by attackers at multiple locations last Friday.
Police also found a Volkswagen Polo with Belgian parking tickets, which led investigators in Belgium to detain individuals who could have been involved in the attacks.
The Wall Street Journal reported that one of the attackers planned to carry out a suicide bombing inside the national stadium to provoke a deadly stampede during the France-Germany match.
It quoted a security guard at the stadium as saying that the attacker was discovered wearing an explosives vest when he was frisked at the entrance to the stadium about 15 minutes into the friendly match.
Thwarted by security guards, the attacker and his accomplice blew themselves up outside the stadium.
Meanwhile, France will intensify its air strikes against ISIS in coordination with the United States, said US Deputy National Security Adviser Ben Rhodes.