The law enforcement agencies could not yet arrest the masterminds behind the gruesome killings of bloggers and freethinkers, although they repeatedly claimed that militant outfits had links with the murders.

With yesterday's murder of Niladri Chattopadhyay, four bloggers have been killed in a span of just seven months. However, investigators have yet to make any visible progress into the killings of bloggers -- Avijit Roy, Oyasiqur Rahman and Ananta Bijoy Das -- except the arrest of a handful of suspects.

Family members of the slain bloggers believe that fatal attacks on freethinkers continue as the government has failed to detect the perpetrators, let alone bring them to book.

They said every time a blogger is killed, law enforcement agencies come up with the statement that militant outfits are behind the killings and that is all.

Seven months have elapsed since the murder of Avijit Roy, a US-based writer and blogger of Bangladesh origin, but law enforcers haven't been able to make any headway in the probe yet.

Detectives investigating the case claimed they have identified seven persons and collected their nicknames and photos but could not arrest anyone yet.

"We are trying our best to find and arrest the killers," said Mashrukure Rahman, deputy commissioner of Detective Branch (South).

He hoped that the case would see some progress once they got the evidence collected by the Federal Bureau of Investigation (FBI). The FBI has been assisting detective police in the Avijit murder case and one of its team already visited Dhaka.

Talking to The Daily Star, Avijit's father Prof Ajay Roy said, "I have also heard of such claims of identifying some people and I do hope that they would be arrested. But I wonder why they are not yet arrested if police have really identified them. Is it their lack of efficiency?" he asked.

Earlier, law enforcers arrested Shafiur Rahman Farabi, a former Hizb ut-Tahrir leader, in connection with Avijit's murder. But they haven't yet been able to establish Farabi's link with the murder.

Avijit was hacked to death near the Dhaka University's TSC on February 26 after he and his wife came out of the Ekushey Boi Mela. His wife Rafida Ahmed Bonya was badly wounded in the attack.

The status of investigation in Ananta Bijoy Das's case is no different.

Blogger Ananta, an activist of Sylhet Gonojagaran Mancha, was hacked to death by a group of assailants in broad daylight at Subid Bazar Bankalapara in Sylhet on May 12. His elder brother filed a murder case on that night.

The case was transferred to Criminal Investigation Department three weeks after the murder, but there hasn't been any progress as none of the killers was detected yet.

A CID team arrested Idris, a photojournalist of the Daily Sabuj Sylhet, as a suspect. But they could not get any breakthrough from him during his remand.

"The investigation is on, but there is no mentionable progress in the case yet," CID Special Superintendent (organised crime) Mirza Abdullahel Baqui told The Daily Star.

Ananta's elder brother Ratneshwar Roy said they only want the killers be arrested as immediately as possible.

Blogger and online activist Oyasiqur Rahman's case, however, saw some progress. Two people --Zikurullah and Ariful Islam -- were arrested while they were fleeing the spot after the attack, but that was also done with the help of locals and a hijra.

Police arrested another suspect Saiful Islam, a madrasa teacher, from a checkpoint in Saidabad area days before the assault on 27-year-old Oyasiqur. He was shown arrested in the Oyasiqur murder case after Zikurullah confessed that Saiful was also part of the group that came to kill Oyasiqur.

But the masterminds in all the cases still remain undetected. Oyasiqur too was hacked to death in broad daylight in the capital's Tejgaon area on March 30.

Law enforcers showed some signs of success only in the case of blogger Ahmed Rajib Haider's case.

They detected the killers and masterminds and pressed charges against the main accused Mufti Jasimuddin Rahmani, chief of militant group Ansarullah Bangla Team (ABT), and seven ABT members and students of North South University.

The case is now under trial.

Rajib, an activist of the Shahbagh movement, was hacked to death in the capital's Pallabi on February 15, 2013.

However, Home Minister Asaduzzaman Khan Kamal claimed the government has not failed to check attacks on bloggers and law enforcement agencies remain active to identify the militant outfits responsible for those killings.

"It is not true that we have failed to stop bloggers' killings. The militants are very desperate but we have arrested many of them. We are also taking action against the others," he told BBC Bengali Service yesterday.

Referring to the slain blogger Niladri Chattopadhyay, the minister said the militants could kill him taking advantage of the time when the people were saying their Juma prayers.

"We hope to identify and arrest the killers as the DB, Rab and intelligence agencies are working to that end," he added.

Kamal said police are ensuring security of the bloggers and other citizens when they ask for it.