Amnesty International has asked the authorities to establish "the fate and whereabouts" of Hasnat Karim who survived the attack and has been missing since being taken in for questioning 11 days ago.
Family members of Tahmid Khan also told AFP that they were in the dark about the 22-year-old Toronto University student's whereabouts after he was taken into custody as part of a police probe into the attack.
Suspected Islamist militants killed 20 diners and two police officers when they raided the upscale Holey Artisan restaurant on the night of July 1. Army commandoes stormed the cafe the next morning, killing all five attackers and rescuing 13 people, including Karim and Khan.
Police have said both were initially interrogated as they tried to piece together what had happened during the siege.
