A U.S. blogger in Bangladesh known for his prominent stance against religious fundamentalism was hacked to death by unidentified attackers in the country’s capital Thursday night, police said.

Avijit Roy and his wife Rafida Ahmed were attacked late Thursday night in Dhaka as they were returning from a book fair at Dhaka University. Ahmed was seriously hurt in the attack.

Roy’s family said he was a prominent voice against religious fanatics and had received threats in the past. It is still unclear who was behind the attack.

The local police chief, Sirajul Islam, told The Associated Press that the assailants used cleavers to attack Roy and his wife, who is also a blogger.

"Several attackers took part in the attack and at least two assailants hit them directly," Islam said, adding that two blood-stained cleavers were found after the attack.

Roy founded a popular blog called Free Mind, which artiles on scientific reasoning and religious extremism were featured. The blog was written in Bengali.

Roy’s brother Anujit said the Bangladesh-born U.S. citizen had returned to Bangladesh earlier this month from the U.S.

Similar attacks have taken place in Bangladesh, a Muslim-majority nation of 160 million people but ruled by secular laws, in the past. Investigators have said religious fanatics were behind those attacks.

In 2013, another blogger, Ahmed Rajib Haider, who also spoke out against religious fanatics, was killed by unidentified assailants near his home in Dhaka.

The Associated Press contributed to this report