Identified by his finger, which was found among the rubble of the Bataclan concert hall, the 29-year-old was one of three men who blew himself up killing 89 people in the bloodiest scene of the carnage.
Born on November 21 1985, in the poor Paris suburb of Courcouronnes, Mostefai's criminal record shows eight convictions for petty crimes between 2004 and 2010, but no jail time.
Paris prosecutor Francois Molins said Mostefai had been singled out as a high-priority target for radicalisation in 2010 but, before Friday, he had "never been implicated in a terrorist network or plot".
Investigators are now probing whether he took a trip to Syria last year, according to police sources.
Friday's attacks, which killed 129 people and wounded 352, including 99 critically, were the worst in Paris's history.
The killer's father and 34-year-old brother were placed in custody on Saturday evening and their homes were searched.
"It's a crazy thing, it's madness," his brother told AFP, his voice trembling, before he has taken into custody.
"Yesterday I was in Paris and I saw what a mess it was."
The brother, one of four boys in the family along with two sisters, turned himself in to police after learning Mostefai was involved in the attacks.
While he had cut ties with Mostefai several years ago, and knew he had been involved in petty crimes, his brother said he had never imagined his brother could be radicalised.
The last he knew, Mostefai had gone to Algeria with his family and his "little girl," he said, adding:
"It's been a time since I have had any news."
"I called my mother, she didn't seem to know anything," he said Saturday.
A source close to the enquiry said Mostefai regularly attended a mosque in Luce, close to Chartres, to the southwest of Paris.