DHAKA, Bangladesh -- Police in Bangladesh announced Saturday they have made additional arrests in the killings of secular bloggers by suspected Islamic radicals that have raised concerns about the government's ability to protect the freedom of speech.

The Criminal Investigation Department arrested two brothers in connection with the killing in May of blogger Ananta Bijoy Das in northeastern Bangladesh, said Special Superintendent Mirza Abdullahel Baqui.

Their role in the killing was not immediately clear. They were described as students, and Baqui said they were arrested on specific information. A photographer of a local daily so far has been the only suspect in that killing.

Das was one of four bloggers who were brutally killed this year.

Dhaka Metropolitan police official Mahbub Alam said two men -- Kausar Hossain Khan, 29, and Kamal Hossain Sardar, 29, -- were arrested late Thursday in the killing of Niladri Chottopadhay Niloy, who was hacked to death on Aug. 7 in Dhaka, the capital.

The suspects are believed to be members of the Ansarullah Bangla Team, an outlawed Islamic group that has been blamed for attacks on atheist bloggers and writers.

Two other people, Saad-al-Nahin and Masud Rana, were arrested two weeks ago for their suspected involvement in Niloy's death. Alam said the latest arrests were the result of the interrogations of the two other men.

Police did not provide more details about Khan and Sardar, but said they had been free on bail after being accused of attempted murder in an attack on another blogger, Asif Mohiuddin, two years ago. Mohiuddin has since fled to Germany.

Niloy was known to his friends as an atheist blogger who criticized radical Islamists at home and abroad, and had sought police protection after receiving threats. Police asked him to leave the country for his safety, they said.

The United States has expressed concern over the killings, while the United Nations called for Bangladeshi authorities to ensure accountability and prevent such violence. U.N. Special Rapporteur Heiner Bielefeldt is to visit Bangladesh starting Monday to assess the state of freedom of religion in the South Asian country.

In February, Bangladeshi-American blogger Avijit Roy was hacked to death on the Dhaka University campus while walking with his wife. Another blogger was attacked and killed in March in Dhaka.

Three suspects including a Bangladeshi-born British citizen have been arrested in connection with Roy's death, but investigations into those cases have made no major breakthroughs.