A prominent Bangladeshi-American blogger known for speaking out against religious fundamentalism was hacked to death in the streets of Bangladesh's capital as he walked with his wife, police said Friday.

The Thursday night attack on Avijit Roy, a Bangladesh-born U.S. citizen, occurred along a crowded stretch of sidewalk as he and his wife, Rafida Ahmed, were returning from a book fair at Dhaka University. Ahmed, who is also a blogger, was seriously injured.

Police have named no suspects in the attack, but Roy was a prominent voice against religious extremism, and his family and friends say he had been threatened in the past for his writings.

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

"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 had founded a popular Bengali-language blog — Mukto-mona, or Free Mind — in which articles on scientific reasoning and religious extremism featured prominently.

Anujit Roy, his younger brother, said Roy had returned to the country earlier this month from the U.S. and was planning to go back in March.

Similar attacks have taken place in Bangladesh, a Muslim-majority nation of 160 million people 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 extremism, was killed by unidentified assailants near his Dhaka home. In 2004, Humayun Azad, a prominent writer and a teacher of Dhaka University, was seriously injured in an attack when he was returning from the same book fair.

Baki Billah, a friend of Roy and a blogger, told Independent TV that Roy had been threatened earlier by people upset at his writing.

"He was a free thinker. He was a Hindu but he was not only a strong voice against Islamic fanatics but also equally against other religious fanatics," Billah said.

"We are saddened. We don't know what the government will do to find the killers. We want justice," he said.