Two years after the deadliest terror attacks in modern French history, the only surviving suspect to take part in the carnage in Paris remains an enigma.
Captured after four months on the run following a massive international manhunt, Salah Abdeslam is now France's most closely guarded detainee.
But the 28-year-old petty criminal-turned-jihadist has not been the goldmine of information investigators have hoped for: he refuses to talk.
Abdeslam's role in the events leading up to November 13, 2015, when Islamic State jihadists armed with guns and bombs killed 130 people enjoying a Friday night out, has yet to be fully understood.
His suicide belt was found in the southern Paris suburbs several days after the bloodshed.
Did he change his mind about blowing himself up, as he told friends before he was finally arrested in Brussels? Or was the device defective?
Abdeslam, a French national of Moroccan origin, is known to have played a key role in organising logistics for the attacks, renting cars and hideouts and helping several jihadists get into Europe via the migrant trail.
And the sprawling investigation has revealed links to the jihadists involved in the Brussels bombings on March 22, 2016 -- four days after his arrest.
What is infuriating for investigators -- and for the families of victims seeking answers -- is Abdeslam's refusal to cooperate since French authorities charged him last year with terrorist murders.
His silence has even prompted his lawyers, Frank Berton of France and Sven Mary of Belgium, to abandon his defence.
Jailed in Europe's biggest prison at Fleury-Merogis, south of Paris, Abdeslam is being held in solitary confinement and monitored around the clock in his cell.
In recent months the conditions of his detention have been slightly relaxed after signs that his mental health was deteriorating.
Justice Minister Nicole Belloubet on Thursday confirmed fears he could commit suicide -- something that the victims' families want to avoid, eager to see him stand trial.
"We are doing everything to make that impossible," Belloubet told LCI television, saying Abdeslam had now been granted a window and limited contact with others.
He has received letters in prison -- sometimes praising his actions -- and responded to some of them, including to a woman whom he reportedly told: "I'm not ashamed of who I am."
Abdeslam surprised many by offering to appear at a separate trial in Belgium for the attempted murder of police officers injured in a shootout at an apartment in the Brussels neighbourhood of Forest, three days before his capture.
He is believed to have fled during the shootout before finally being captured across the city in Molenbeek, where he once ran a bar with his brother Brahim, one of the Paris suicide bombers.
The Brussels trial is due to take place from December 18-22, with a Paris appeals court expected to rule soon on whether to give the greenlight for Abdeslam's temporary transfer.
It remains to be seen whether he will break his silence at the trial, and some of the victims' lawyers fear he will use it as an opportunity to avoid being sent back to France.
"Why would this guy, who isn't talking to investigators in France, demand to be sent to trial in Belgium?" said Gerard Chemla, lawyer for some of the plaintiffs.
"We should ask ourselves 'is this is an attempt to escape from French authorities?'"
A date for Abdeslam's French trial -- expected to be one of the most closely watched in decades -- has not yet been set.