The bomb maker for attacks on Paris and Brussels had a previous career as a jailer for the so-called 'Islamic State' group, a lawyer for one of his hostages has said.
Brussels authorities say Najim Laachraoui, a 24-year-old Belgian, built the explosive belts used in the assaults on both European capitals and blew himself up at Brussels Airport on March 22.
He was also one of several jihadis who held some hostages captured in Syria, according to Marie-Laure Ingouf, who represents two former French captives.
"Laachraoui was one of the jailers of the French ex-hostages, and of other decapitated hostages, as they all shared the same cell," she said in a statement.
Her statement, which also referred to beheaded American captives James Foley and Steven Sotloff, said Laachraoui was known at the time as Abou Idriss and worked alongside Mehdi Nemmouche, a Frenchman accused of a 2014 attack on the Brussels Jewish Museum.
Nemmouche had already been identified as a jihadi jailer in Syria by one of Ms Ingouf's clients, Nicolas Henin.
Her statement partially confirms reports in French and British media identifying Laachraoui as the man who first imprisoned Mr Foley and British photojournalist John Cantlie and played a key role during negotiations over the fate of Mr Henin and three other French journalists.
All the French journalists including Mr Henin were eventually released. Mr Foley was beheaded in one of IS's first gruesome videos, while Mr Cantlie last appeared in a March propaganda video. His current whereabouts are not known.
Further details about the alleged roles played by Laachraoui or Nemmouche in the captivity of Mr Foley and Mr Sotloff were not immediately available. Ms Ingouf declined to say anything beyond the statement.
Laachraoui, a young mechanical engineering graduate, left for Syria in February 2013 - an early recruit amid a wave of Belgians who have travelled to fight with the extremists there - and later returned home under the alias Soufiane Kayal.
A central player in the dual attacks that killed a total of 162 people in Brussels and Paris, Laachraoui's job was to make the explosives and the suicide vests used in the assaults. His DNA was found on one of the vests that detonated inside Paris's Bataclan concert hall as well as one that blew up outside France's national stadium on November 13.
The last publicly distributed photograph of Laachraoui shows him pushing a cart full of explosives at Brussels Airport moments before setting them off.