FILE- In this April 29, 2016 file photo, a Bangladeshi man holds a portrait of a University Professor A.F.M. Rezaul Karim Siddique during a protest against his killing in Dhaka, Bangladesh. Police have arrested four members of a banned militant group, including a regional commander, suspected in the killing last month of the university professor in northwestern Bangladesh, officials said Tuesday. (AP Photo, File)

DHAKA, Bangladesh - Police have arrested four members of a banned militant group, including a regional commander, suspected in the killing last month of a university professor in northwestern Bangladesh, officials said Tuesday.

English professor A.F.M. Rezaul Karim Siddique was hacked to death on April 23 on his way to work at a state-run university in the city of Rajshahi. The attackers used sharp weapons and quickly fled the scene.

Police tracked down the regional commander of the group Jumatul Mujahedeen Bangladesh on Sunday night in the northern district of Bogra. The suspect, Maskawath Hasan Sakib, nicknamed Abdullah, confessed involvement in the attack before a magistrate, leading to the arrests of the three other suspects on Monday night, according to Rajshahi Metropolitan Police Commissioner Mohammed Shamsuddin.

"Of the arrested four, three directly took part in the killing while the fourth man was waiting with a motorbike they used," Shamsuddin said.

He would not disclose the identities of the three others detained "for the sake of the investigation," but said the motorbike had been recovered.

Abdullah told police during his interrogation that the attackers did not know why Siddique was targeted, and that they had carried out the killing on orders from their superiors, Shamsuddin said.

Family members described Siddique as a quiet man who led a cultural group and edited a literary magazine. His killing was similar to other attacks over the last two years by suspected Islamist militants that targeted atheist bloggers, intellectuals and rights activists in Muslim-majority Bangladesh.

The Islamic State group claimed responsibility for Siddique's killing and accused him of advocating atheism, according to the SITE Intelligence Group, which monitors jihadi websites. But authorities rejected the claim, denying that the group has any presence in the country.

At least three other professors at Rajshahi University have been killed in recent years, allegedly by Islamist groups.

Prime Minister Sheikh Hasina's government has cracked down on domestic militant groups, and accuses the political opposition of supporting religious radicals in retaliation for the government's prosecution of people suspected of committing war crimes during the country's 1971 independence war.

The main opposition Bangladesh Nationalist Party and its main ally the Jamaat-e-Islami party have denied any role in the attacks.

News from © The Associated Press, 2016