A Bangladeshi politician has spoken of his shock on discovering his son was one of the suspected gunmen who murdered foreign hostages, and said many young men from wealthy, educated families had gone missing.

Imtiaz Khan Babul said he was ‘stunned’ to learn of his son Rohan's involvement in the attack, and that he believed young Bangladeshi men were becoming radicalised online.

His comments came after Bangladesh's home minister said the attackers who stormed the upmarket cafe on Friday night, taking dozens of diners hostage and killing 20, were highly educated and from wealthy families.

‘We never imagined this,’ said Babul, an official with the ruling Awami League party, in an interview with the BBC.

‘There was nothing at home, no books or anything to indicate that he was leaning that way. So we had no inkling.’

Babul, whose son was among those killed when Bangladeshi security forces stormed the cafe, said he had shared his concerns with friends in Dhaka.

‘When I was searching for my son I found that many other boys are missing. Well-educated boys from good, educated families, children of professionals, government officers,’ he said.

‘I used to share my sorrows with them. We do not know how this is happening.’

Six young men were shot dead Saturday at the end of the all-night siege in a Dhaka cafe claimed by the Islamic State group. One was taken alive and is being questioned.

One of those killed may have been an innocent bystander, but among the remaining five are a graduate of Bangladesh's leading private university, an 18-year-old student at an elite school, and Rohan, Babul's son.

Another was a former madrasa student from the northern district of Bogra which is seen as a hotbed of Islamist radicalism.

The government has said all the attackers were members of the Jamaeytul Mujahdeen Bangladesh (JMB), a banned local Islamist group.

- Fifth attacker identified -

Rohan reportedly studied at Monash University in Malaysia after leaving Scholastica, where his mother teaches.

Babul said he could only guess at how wealthy young Bangladeshis were becoming radicalised, but that believed it could be through the Internet.

On Tuesday, police said they had identified a fifth attacker as Shafiqul Islam Uzzal, a 26-year-old from Bogra who had been working as a kindergarten teacher in Dhaka.

‘His father and brother identified him by seeing the photos of his dead body. They said they had no idea how he joined the JMB,’ Bogra police spokesman Gaziur Rahman told AFP.

‘They said Uzzal left home on January 2, saying that he was going to a pilgrimage. And he never returned,’ said Rahman.

Witnesses say the perpetrators of the attack spared the lives of Muslims while herding foreigners to their deaths, killing many with machete-style weapons.

They included nine Italians, seven Japanese, a US citizen and a 19-year-old Indian student.

On Tuesday the assistant commissioner of Dhaka police Rafiqul Islam told AFP police had filed an anti-terrorism case against five known Islamists and an unspecified number of ‘unidentified attackers’, without giving further details.