By Ruma Paul
DHAKA (Reuters) - Attackers armed with machetes killed a blogger in Bangladesh on Friday, the fourth such killing of an online critic of religious extremism in six months, prompting calls by human rights groups for a swift and thorough investigation.
Militants have targeted secularist writers in Bangladesh in recent years, while the government has tried to crack down on hardline Islamist groups seeking to make the majority-Muslim South Asian nation of 160 million people a sharia-based state.
Niloy Chatterjee, 40, an advocate of secularism, was killed in his flat in the capital Dhaka, said police official Mustafizur Rahman.
"We are speechless. He was demanding justice for the killing of other bloggers," said Imran Sarker, head of a network of activists and bloggers.
"Who will be next for demanding justice for Niloy?"
Chatterjee, who used the pen-name Niloy Neel, was a critic of religious extremism that led to bombings in mosques and the killing of civilians, Sarker said.
Chatterjee was also one of hundreds of bloggers driving a movement demanding the death penalty for Islamist leaders accused of atrocities in Bangladesh's 1971 war of independence.
Prime Minister Sheikh Hasina opened an inquiry into war crimes in 2010.
A tribunal has since convicted several senior leaders of an Islamist party, who in 1971 opposed the breakaway of Bangladesh, then known as East Pakistan, from Pakistan, of various crimes.
They denied wrongdoing.
U.N. human rights investigators and rights groups condemned Friday's killing and called for a thorough investigation.
"The violent killing of another critical voice in Bangladesh shows that serious threats to freedom of expression persist in the country," U.N. special rapporteur on freedom of expression David Kaye and the special rapporteur on extrajudicial killings Christof Heyns said in a joint statement issued in Geneva.
Amnesty International urged Bangladesh to send a strong message that killings aimed at silencing dissenting voices were despicable and would not be tolerated.
"Thorough, effective, independent and impartial investigations must be carried out promptly to ensure that all those responsible are brought to justice in fair trials without recourse to the death penalty," Amnesty said in a statement.
The Committee to Protect Journalists (CPJ) also appealed to the Hasina government to solve "this barbaric murder".
In February, machete-wielding assailants killed a U.S. citizen of Bangladeshi origin and critic of religious militancy, Avijit Roy, and seriously injured his wife and fellow blogger, Rafida Bonya Ahmed, after they left a book fair.
On March 30, Washiqur Rahman, another secular blogger who aired his outrage over Roy's death on social media, was killed in a similar fashion. Another secular blogger, Ananta Bijoy Das, was hacked to death on May 12.
The Indian-born head of al-Qaeda in the Indian subcontinent claimed responsibility in May for a series of attacks in Bangladesh and Pakistan, including Roy's. The United States said it was unable to confirm the claim.
(Additional reporting by Stephanie Nebehay in Geneva; Editing by Gareth Jones)