DHAKA, Bangladesh — When he was released with seven other hostages after a harrowing night of terrorist violence last Saturday, Tahmid Hasib Khan expected to be greeted with welcoming arms. Instead, Mr. Khan, a 22-year-old University of Toronto student, was grabbed by the Bangladeshi police, beaten and taken into custody. He has not been seen since.

Mr. Khan has been held largely incommunicado by law enforcement in Bangladesh’s capital city of Dhaka for the past week, suspected by the police of being involved in the attack in which gunmen carrying explosives stormed a Dhaka restaurant and the Islamic State took credit. Mr. Khan’s family and friends say it is a case of mistaken identity, but as the days have passed, they have grown increasingly desperate to make contact with him, fearing for his life in a country where brutal interrogation practices are commonplace.

“We don’t want anything bad to happen to him,” said a cousin of Mr. Khan’s, Ali Faiyaz Shoumo. “We just want to know that he’s alive and want him to get proper medical attention and legal representation.” They said Mr. Khan has epilepsy, a condition that is aggravated by stress.

Human rights experts share their concerns. “The Bangladesh security forces have a long record of using beatings and other forms of torture to try to extract confessions from suspects,” said Phil Robertson, deputy Asia director of Human Rights Watch. “Bangladesh has been playing a desperate game of catch-up to try to figure out who is behind this attack, and the worry is that this will encourage use of extreme methods like torture against these suspects.”