Mr. Caillet met Fabien Clain during that period, in Egypt, and described him as a “modest, affable individual open to debate.”
In Artigat, the Clains grew close to another prominent French jihadist, Abdelkader Merah, who was convicted in 2017 of being part of a criminal terrorist conspiracy. Mr. Merah’s younger brother, Mohamed, killed seven people in Toulouse in 2012, including four at a Jewish school, before the police killed him.
In January 2015, just before leaving for Syria, Fabien Clain bought $4,000 worth of music equipment, including a microphone, voice changing equipment and a license to Pro Tools, the music editing software, from a shop in Toulouse, according to French media reports.
“He was a ghost whose name could be found in almost all jihadi cases, a figure who fascinated the whole French jihadi sphere,” said Guillaume Denoix de Saint Marc, the president of the French Association of Victims of Terrorism, or A.F.V.T.
Mr. Denoix de Saint Marc said he would have preferred that Mr. Clain be captured and interrogated about his involvement in the Paris attacks.
“Fabien wasn’t very talkative, but his brother Jean-Michel usually had more to say,” said Chantal Anglade, an A.F.V.T. member whose daughter was wounded in a Cairo bombing in 2009, for which the Clain brothers were interrogated.
In 2009, Mr. Clain was sentenced to five years in prison for his role, as part of the Artigat cell, in recruiting radicalized people and sending them to Iraq. He was released in 2012 and is believed to have left for Syria to join the Islamic State around March 2015.
“He embodied jihadism with a capital J,” Ms. Anglade said. “We can’t be relieved about someone’s death, but we only have contempt for Mr. Clain.”