Imtiaz Khan Babul said his 22-year-old son Rohan Imtiaz, who was killed when commandos stormed the cafe on Saturday, had been a top-scoring student whose behaviour gave no hint that he was radicalised before he disappeared last December.
“I was stunned and speechless to learn that my son had done such a heinous thing,” a tearful Babul told the AFP news agency.
“I don’t know what changed him. There was nothing that would suggest that he was getting radicalised. He hardly read any religious books.”
Babul, an official with the ruling Awami League party, said he had not seen his son since travelling to India in December with his math teacher wife, leaving the couple’s three children in Dhaka.
In the months that followed Rohan’s disappearance, Babul lobbied senior party officials to help find his only son and even scoured the city’s morgues.
As he searched, he met other families who had suffered the same fate.
“Even yesterday, one of them was saying that I was lucky that I got the body of my boy. Some of them are not so lucky,” he said.
Babul said he believed his son might have been “brainwashed” on the Internet.
Bangladesh’s Home Minister has said that the men behind Friday night’s attack at the upmarket cafe, which left 22 people dead, were highly educated and from wealthy families.
Witnesses said the perpetrators of the attack, which the Islamic State group has claimed responsibility, spared the lives of Muslims while herding foreigners to their deaths, killing many with machetes.
The victims included nine Italians, seven Japanese, a US citizen and a 19-year-old Indian student.
Six young men were shot dead at the end of the all-night siege. One was taken alive and is being questioned.
The government has alleged that all the attackers were members of the Jamaeytul Mujahdeen Bangladesh, a banned local Islamist group.
Rohan had enrolled at Monash University Malaysia with Nibras Islam, another of the dead militant, in 2012, according to a former student at the university.