Niloy Chowdhury, an atheist and outspoken critic of religious extremism, was killed Friday in Bangladesh when assailants entered his apartment building posing as potential tenants, marking the fourth such murder in Bangladesh since the start of 2015, the Bangladesh Blogger and Activist Network reports.

"They entered his room in the fifth floor and shoved his friend aside and then hacked him to death. He was a listed target of the Islamist militants," the network's head Imran H. Sarker told the AFP news agency, according to Al Jazeera. 

The 40-year-old blogger, who went by the name Niloy Neel on Facebook, was known for his criticism of religious extremism that led to bombings in mosques and the killing of numerous civilians both at home and abroad.

Chowdhury's wife was at home during the attack, but she was kept "confined" while he was killed in another room, according to the New York Daily News.

No group has claimed responsibility for the attack.

This is the fourth incident where a blogger in Bangladesh was killed by "Islamist militants" this year, according to ABC News.

In Feburary, Avijit Roy was hacked to death on the Dhaka University campus while walking with his wife. In March, Washiqur Rahman Babu was hacked to death in Dhaka by three men over alleged anti-Islamic writings. Lastly, on May 12, Ananta Bijoy Das, was killed in the Subid Bazar area of Sylhet city.

In all four of these cases, authorities note that the link between the murders and "radical Islamist groups" are "under investigation".