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The journalists who were freed in April 2014 came to the Belgian capital to testify about the character of Nemmouche, a 33-year-old Frenchman who faces a life sentence if convicted of the murders.
"I have absolutely no doubt about the fact that Mehdi Nemmouche who is present here was my jailer and torturer in Syria under the name of Abu Omar," former hostage Nicolas Henin told the trial.
His colleague Didier Francois also said he "had no doubt" Nemmouche had held him hostage along with fellow Frenchmen Edouard Elias and Pierre Torres who were not present Thursday.
Francois said Nemmouche hit him with "around 40 blows of a truncheon" among other abuses during the time the journalists were held by the Islamic State in the hospital turned prison.
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He added that the violence and "tortures" meted out by Nemmouche mainly targeted Syrians and Iraqis also held there.
The four French journalists were kidnapped in June 2013 and held hostage by the IS in the northern city of Aleppo until April 2014.
The prosecution and a lawyer for Jewish groups asked the journalists to attend the trial to show what Nemmouche had done in Syria between January 2013 and February 2014.
Nemmouche's lawyers charged that their testimony amounts to a "stunt" and a "trial within a trial" because their kidnapping is the subject of separate proceedings in France.