Four French journalists kidnapped and held in Syria from 2013 to 2014 had identified a guard known as 'Abou Idriss', sources close to the investigation said on Friday.
One of the journalists, Nicolas Henin, "has formally identified" Abou Idriss as being Najim Laachraoui, his lawyer Marie-Laure Ingouf said, confirming reports in French newspapers.
Belgian prosecutors have said Laachraoui travelled to Syria in February 2013 to join up with IS armed group. There was no further trace of the Belgian national until he was registered under a false name at the border between Austria and Hungary in September 2015.
Laachraoui, 24, was one of the two suicide bombers who struck Brussels airport on 22 March, while a third attacker blew himself up at on a metro train, with the two attacks killing 31 people.
Prosecutors have also linked him to November's attacks in Paris in which 130 people died, saying his DNA was found on a suicide vest and a piece of cloth at the Bataclan concert hall where 90 people were killed.
The former French hostages have already identified two Frenchmen as being among their jailers when they were held in Syria.
One, Mehdi Nemmouche, is in custody accused of killing four people in an attack on the Jewish museum in Brussels in 2014.
The other is Salim Benghalem, who has been sentenced in absentia in France for recruiting for IS and is listed as a "foreign terrorist combatant" by the United States.
- with AFP