A Bangladesh court sentenced four extremists to hang Wednesday for their fatal machete attack on a celebrated writer, in a case that took nearly two decades to reach a verdict.

The men brutally maimed award-winning author and language professor Humayun Azad outside a book fair in 2004 -- the first in a wave of violent attacks on free speech advocates in the Muslim-majority country. 

Images of a blood-soaked Azad after the attack shocked the country, and a legion of fans mourned when the 56-year-old died several months later while seeking treatment in Germany.

Two of the attackers are still at large, and a fifth member was shot dead by police in 2014 after reportedly attempting to flee a prison van.

"Four were handed down the death penalty over the murder including two who were sentenced to death in absentia," Abdullah Abu, chief prosecutor in the capital Dhaka, told AFP. 

The perpetrators were members of Jamayetul Mujahideen Bangladesh (JMB), a banned outfit whose leader ordered Azad's murder after the author penned a book that mocked and criticised fundamentalists. 

The organisation was also responsible for a series of deadly bomb blasts around the turn of the century, and several senior members were executed in 2007. 

There was no immediate comment from Azad's family after the verdict. His son moved to Germany after allegedly facing online threats.   

But relatives have expressed dismay over the slow pace of the trial.

"I've no interest in the verdict. What's the point of a verdict after 18 years?" Azad's brother Manjur Kabir told online news portal Bdnews24 on Tuesday. "I'm not in a position to feel happy or have regrets. We don't want justice or a verdict anymore."

Azad's widow "feels the same", he added. 

The writer's murder was followed years later by a series of fatal machete attacks on secular and atheist writers as well as gay rights activists by a JMB offshoot.

Eleven years to the day after the attack on Azad, US-based writer Avijit Roy was hacked to death as he was leaving the same book fair.

Bangladesh has since launched a nationwide crackdown on extremist groups, killing more than 100 militants in raids across the country and arresting more than 1,000 suspected extremists.