Writing on the website of his former employer, Le Point magazine, journalist Nicolas Henin said Mehdi Nemmouche, who has been extradited to Belgium and held for questioning, was his jailer between July and December 2013.
One of a group of four journalist hostages freed in April, Henin said the 29-year-old, who spent more than a year fighting in Syria, was a feared and violent figure.
"When Nemmouche was not singing, he was torturing," wrote Henin. "He was part of a small group of Frenchmen whose visits would terrify the 50-odd Syrian prisoners held in the cells nearby."
"Every night the blows would start raining down in the room, where I was also interrogated. The torture lasted all night, until dawn prayers."
Interior Minister Bernard Cazeneuve confirmed that French intelligence services had "transferred elements to the judiciary that suggest (Nemmouche) may have been the jailer of our hostages," following a report in Le Monde newspaper.
Le Point said that Henin had kept quiet on Nemmouche's role in his detention, to protect the Western hostages still being held by Islamic State militants, but decided to speak out following Le Monde's report.
Henin's lawyer Marie-Laure Ingouf told the French news agency AFP that "Nemmouche was one of his jailers."
"All the hostages confirm this. They lived alongside him for several months."
The Frenchman of Algerian descent was remanded in custody in early August on charges of "murder in a terrorist context" over the May 24 shooting, which left four dead.
He is scheduled to appear on September 12 before a Brussels judge who will decide whether to extend his preventative detention.
The attack in broad daylight left an Israeli couple, a Frenchwoman and a Belgian man dead, and raised fears of terror attacks from foreign fighters returning from Syria.
Nemmouche was arrested in the southern French city of Marseille days afterwards.
He has been sentenced seven times in France, including for armed robbery, and has spent seven years in jail where he was notably found proselytising Islam.