Foued Mohamed Aggad had travelled to war-torn Syria with his brother and a group of friends at the end of 2013, according to a source close to the investigation.
The two other attackers involved in the massacre of 90 concertgoers at the Bataclan -- Omar Ismail Mostefai, 29, and 28-year-old former Paris bus driver Samy Amimour -- had also been in Syria.
Two of the gunmen blew themselves up with suicide belts packed with explosives after the killing spree, the worst of the November 13 Paris attacks.
Most of the group of six men from Strasbourg who went to Syria with Aggad were arrested in the Meinau area of the city on their return in May last year and are all in custody on terrorist charges.
But Aggad stayed on in Syria, the source said.
Investigators believe two brothers from the group, Mourad and Yassine Boudjellal, were killed fighting with the Islamic State in Iraq and the Levant (ISIL) before it restyled itself as the Islamic State (IS) group.
When questioned on their return, the men claimed they had been horrified by what they had witnessed in Syria and had started to dribble back to France from February 2014.
Aggad was identified at the end of last week after his DNA was matched with members of his family, the police source said.
Police suspect the Strasbourg group had been recruited by Mourad Fares, 31, who was known to French intelligence for recruiting jihadist fighters through social media and the Internet.
Fares -- who France's Interior Minister Bernard Cazeneuve has described as a "particularly dangerous individual" -- was arrested in August 2014 in Turkey before being handed over to the French authorities.
Nearly 1,500 people were watching the Californian band Eagles of Death Metal play at the Bataclan when the gunmen burst in last month, leaving 90 dead and hundreds hurt, mostly people under 40.
