A Toronto university student who survived a militant attack in Bangladesh has been detained by authorities in the country's capital since the weekend siege, his family said Wednesday as they called for the young man's release.

Police have not explained why they've held Tahmid Hasib Khan, 22, for the past four days and concerns are mounting for his well-being, his family said.

"We are concerned that they might put a case against him," his cousin Rasheek Irtisam said from Dhaka. "If they put a case against him, it will take years in Bangladesh."

Khan, a permanent resident of Canada, is an undergraduate student studying global health at the University of Toronto. His family said Khan arrived in Dhaka on July 1 to celebrate Eid with his family, and planned to travel to Nepal to begin an internship next week.

They said Khan was meeting friends at a Dhaka cafe when he and others were taken hostage for 10 hours on July 1.

Two police officers and 20 hostages -- nine Italians, seven Japanese, an Indian and three students from American universities -- were killed in the siege at the upscale restaurant in Dhaka's diplomatic zone. Six of the attackers also died.

Authorities held five former hostages for questioning but have released three.

Khan's father was able to speak with his son twice since the siege ended, with the last phone call taking place two days ago.

"He said he was alive, he was doing fine," said Irtisam, adding that his cousin had epilepsy and sometimes suffers seizures when under extreme stress. "We are right now worried about that. We are trying to get some doctors over there."

The family have also reached out to Bangladesh's Home Minister in an attempt to secure their son's release and have a lawyer stationed outside the building where Khan is being held, Irtisam said.