DHAKA, Bangladesh -- A publisher of secular books was hacked to death and three other people wounded in fresh attacks in Bangladesh's capital that were claimed by Muslim radicals, and a human rights group called on the government to urgently protect freedom of expression.

The latest violence on Saturday followed the killings of four atheist bloggers this year, while the Islamic State group has claimed responsibility for three other attacks.

Two of the new victims had published works of Bangladeshi-American blogger and writer Avijit Roy, who was hacked to death on the Dhaka University campus while walking with his wife in February.

The local Islamist group Ansarullah Bangla Team had claimed responsibility for the blogger killings and recently threatened to kill more bloggers.

Another militant group, Ansar al-Islam, the Bangladesh division of Al-Qaida in the Indian Subcontinent, issued a statement claiming responsibility for the attacks on publishers Faisal Arefin Deepan and Ahmed Rahim Tutul, according to the SITE Intelligence Group, which monitors jihadist online postings. The claim of responsibility could not be independently verified.

Amnesty International called on the Bangladesh government to "act urgently" to ensure the protection of others in the country, calling the attacks " a deliberate assault against freedom of expression."

The body of Deepan of the Jagriti Prokashoni publishing house was found inside his office following the second of Saturday's attacks, senior police officer Shibly Noman said.

Earlier in the day, publisher Ahmed Rahim Tutul and two writers were shot and stabbed by three men in the office of the Shudhdhoswar publishing house, said police officer Abdullah Al Mamun.

Police chief Jamal Uddin Meer said the assailants then locked the wounded men inside the office before escaping. "We had to break the lock to recover them," Meer said.

The two writers were identified by police as Ranadeep Basu and Tareque Rahim. All three of the victims were hospitalized, and Tutul was in critical condition, Meer said.

Ansar al-Islam accused the "secular and atheist publishers" of putting out books by blasphemers that dishonoured the Prophet Muhammad, and threatened more attacks.

Bangladesh has been rocked by a series of attacks this year claimed by Islamic extremists, including the blogger killings and, more recently, the killings of two foreigners -- an Italian aid worker and a Japanese agricultural worker. An Oct. 24 bomb attack on thousands of Shiite Muslims in Dhaka killed a teenage boy and injured more than 100 other people.

The Islamic State group claimed responsibility for the attacks on the two foreigners and the bombing, but Bangladesh's government has rejected that the extremist Sunni militant group has any presence in the country.

The government has instead blamed domestic Islamist militants along with Islamist political parties -- specifically the opposition Bangladesh Nationalist Party and its main ally, Jamaat-e-Islami -- for orchestrating the violence to destabilize the already fractious nation.