Fourteen people are going on trial over the 2015 attacks against the satirical newspaper Charlie Hebdo and a supermarket in Paris that sparked a wave of violence by the so-called Islamic State group in Europe.
Seventeen people and all three gunmen died during the three days of attacks in January 2015 in the French capital.
Those on trial, 13 men and one woman, in France's terrorism court in Paris are accused of buying weapons, cars, and helping with logistics.
Most say they thought they were helping plan an ordinary crime.
Three, including the only woman to face charges, are being tried in absentia after leaving to join IS.
What happened in the 2015 attacks?
The attacks from January 7-9 2015 started during an editorial meeting at Charlie Hebdo, whose offices had been unmarked and guarded by police since the publication of caricatures of the Prophet Muhammed years before.
Brothers Cherif and Said Kouachi gunned down 12 people before carjacking a vehicle and fleeing.
They claimed the attacks in the name of al-Qaida.
Two days later, on the eve of the Jewish Sabbath, Amedy Coulibaly stormed the Hyper Cacher supermarket - killing four hostages in the name of IS as the brothers took control of a printing office outside the French capital.
The attackers died that day during police raids.
It took days for investigators to realise that Coulibaly was also responsible for the seemingly random death of a young policewoman the previous day.
It took further weeks to unravel the network of petty criminals and neighbourhood friends linking the three attackers.
By then, Coulibaly’s wife had left for Syria with the help of two brothers also charged in the case.
Most of the 11 who will appear in court insist their help in the mass killings was unwitting.
"Since 2012, terrorism capitalised on the prevailing delinquency there is around these terrorists," said Samia Maktouf, a lawyer for one of the attack survivors.
"They are not second fiddles, they are full accomplices. You know, when you provide a weapon it’s not to go and party."
Later that year, a separate network of French and Belgian fighters for IS struck Paris again, this time killing 130 people in attacks at the Bataclan concert hall, the national stadium, and in bars and restaurants.
Wednesday’s trial is opening under tight security - with multiple police checks for anyone entering the main courtroom or the overflow rooms.
At nearby newsstands, the latest issue of Charlie Hebdo appeared, reprinting the caricatures of the Prophet Mohammed cited by the gunmen who killed so many of the publication’s editorial staff.
"They died so that you journalists could do your jobs," said Richard Malka, the lawyer for Charlie Hebdo.