Mostefai, Parisians learned, was identified through a gruesome find. He was literally "fingered" by a fingerprint, pulled from a severed finger, found in the carnage of body parts in the concert hall, according to investigators.
The series of terrorist attacks left at least 129 dead and scores more wounded, many of them still in critical condition.
Perhaps as shocking as the crimes committed was where Mostefai had been a resident. Far from being any sort of poor or impoverished area, Chartres is a scenic tourist spot, with narrow cobbled streets and stunning vistas. It is about 50 miles southwest of Paris.
Even in the neighborhood where Mostefai lived, just 10 minutes from the center of town in the La Madeleine area, the streets are quiet and tree-lined, with clean streets and sidewalks.
People in Chartres wonder: Why here?
So how did Mostefai, previously unknown to the general public, become radicalized to do such a thing?
"Yes, that is what shocks people," said Mayor Jean-Pierre Gorges, who was the first person to publicly identify Mostefai, by name, in a Facebook entry late Saturday. The mayor is a member of Parliament and is a well-known figure in the region.
"We have someone who was radicalized in a town that's practically the capital in France of Catholicism -- and, you saw the cathedral, one of the world's greatest historic monuments."
Francois Molins, the lead French prosecutor in the investigation, said the assailant "was identified thanks to his finger, and fingerprints." He said Mostefai, however, had been known to authorities as a possible threat.
Molins said in a press conference that Mostefai had an "S" file on him for years, which means investigators believe he had been "radicalized" in some way, though it was not clear whether he would act on his radicalization.
Surveillance files have been opened on more than 5,000 suspected Islamic extremists in France, but security services only have the manpower and resources to monitor a small fraction of these numbers 24/7. About 1,000 have traveled to Syria and Iraq to fight jihad or are in transit there -- and those are just the ones French authorities know about.
Gorges said in an interview with CNN on Sunday that he decided to reveal Mostefai's identity publicly after a briefing by investigators.
Mosque denies mayor's claims of radicalization
Mostefai had become radicalized at a small mosque on the edge of Chartres, in the neighborhood known as Lucé, the mayor said.
Gorges said he had been informed that Mostefai some four to five years ago was "radicalized" at the mosque located on "Paradise Street." The mosque is a small, unimposing building looking more like a former house in a suburban neighborhood.
But officials at the mosque said the mayor was wrong.
Standing in a throng of French reporters, Karim Benaya said that he had no knowledge of Mostefai, and that neither he nor his family were members or attended.
"It's completely false," said Benaya, the vice president of the mosque association. Standing beside him was Abdallah Benali, the president of the association, who agreed.
But the two men said that they have been at the mosque only since 2013. And the mayor said he was told Mostefai had become more extreme in the years before that, and that it had been because of a "visiting imam."
"In greater Chartres -- there's a number of places of worship," the mayor said. "And there are some which are run correctly. But there are others where there are Imams who try to convert people -- which leads to radicalization."
Prosecutors could not be reached to answer the mayor's claims. But much of what Gorges said had been detailed publicly on Saturday by the lead prosecutor, and numerous French publications reported having numerous sources on the question.
In the small, neat suburban neighborhood where Mostefai had lived for years, before apparently leaving around 2012, neighbors preferred not to be identified. They told reporters that Mostefai had not been the only terrorist in that area.
A neighbor said another jihadi had gone to Syria to fight and had been killed. The two had, he told CNN, overlapped in the area, and could have provided another avenue for "radicalization."
The neighbor did not provide the alleged neighbor's name, and CNN could not independently confirm the details of that second jihadi. (But they may have referred to Sofiane Sankawi, who was from the area and whose death in Syria as a fighter earlier this year was widely reported.)
Not considered especially religious as child
Mostefai's childhood home is in Courcouronnes, a suburb of Paris, about 40 miles from Chartres. It is considered a lower- to middle-class area, though it is not known for any serious radicalism or extremism.
Mostefai was a "normal" child who played soccer and sported long hair as a teenager, two of his childhood friends told CNN.
Dem, a childhood friend who wants to be identified only by his first name out of privacy concerns, described Mostefai as not especially religious.
The friend, wearing a sweatshirt, sneakers and Islamic thawb, or robe, spoke on a manicured street outside the mosque in the Courcouronnes, where Mostefai was born, according to the Paris prosecutor.
Mostefai went to the mosque south of Paris' city center "every now and then" -- sometimes with his parents, sometimes alone, Dem said.
He was the middle of three brothers and had two sisters, according to Dem and the mosque director. The bomber moved to Chartres while an adolescent, his friend said.
It had been four or five years, Dem said, since he'd last seen Mostefai.
Another man in Courcouronnes, described the bomber as a "very, very, very close friend," though it had also been several years since he'd last seen him.
"I am shocked," Abdel Afid said. "I don't understand."
Dem said when he last saw Mostefai, he'd been struck by how much had had matured, now with a wife and little daughter.
Chartres Mayor Gorges said that he is truly worried after the recent attacks, which come 10 months after the terrorist strike at the Charlie Hebdo newspaper in Paris. The mayor's concerns are what many French citizens, and people from around the world, are perhaps wondering.
"We have 4,000 people who are deemed so dangerous they could potentially do the exact same thing (as Mostefai)," he said. "And what are we doing? What does the government propose doing? Nothing."
Now, two days after the attacks, the identities of the other attackers are only just beginning to come to light. And even now, there is still no photo of Mostefai that has emerged.
Agnes Thibault-Lecuivre, the Paris prosecutor's spokeswoman, told CNN that six people -- all family relatives of Mostefaï -- have been detained. They may be providing investigators the leads that everyone is looking for.