The sister of slain Rajshahi University professor Rezaul Karim Siddiquee mourn after hearing news of his death at Salbagan, Rajshahi in Bangladesh 23 April 2016.(Abdullah Iqbal/EPA)

The Islamic State has claimed responsibility for the brutal death of a university professor in Bangladesh, who they allegedly targeted for his "call to atheism."

Rezaul Karim Siddique, 58, taught English at Rajshahi University in Bangladesh's north-west. He was on his way to work when he was attacked by militants wielding machetes, who hacked him to death.

Siddique's neck was hacked at least three times and was 70 percent to 80 percent severed, Mohammad Shamsuddin, Rajshahi's police commissioner, said. "By examining the nature of the attack, we suspect that it was carried out by extremist groups."

Deputy Police Commissioner Nahidul Islam told the Guardian that Siddique was involved in a number of cultural programs and had founded a music school in Bagmara in the Rajshahi district — which in the past has been a notorious hotbed of religious extremism.

IS claimed responsibility for the attack through their mouthpiece Amaq news Agency, the SITE intelligence group reported.

Students from Rajshahi University protested Siddique's killing, demonstrating along main thoroughfares, and blocking traffic. Siddique's wife, Hosne Ara, said she couldn't understand why someone would want to kill her husband. "As far as I know, my husband didn't have any personal enmity with anyone," Ara told the BBC. "I can't believe that he has been murdered."

In 2015, the rising tide of religious extremist violence began sweeping Bangladesh. Last September, an Italian aid worker was killed by unidentified assailants in Dhaka, the nation's capital. Just days later, a Japanese national was murdered in northern Bangladesh. IS claimed responsibility for both killings.

During a press conference after the two murders, Bangladesh Prime Minister Sheikh Hasina hastily ruled out the possibility that IS were active in Bangladesh. "Until now, IS or global terror groups like it have not been able to operate in Bangladesh," Hasina said. "Our intelligence agencies are active... we will not allow any such activities in Bangladesh."

In the following months, the Sunni terror group claimed responsibility for a number of other attacks. In October, IS claimed responsibility for a series of explosions targeting Shia Muslims in Dhaka during a religious procession, killing one person. In November, militants stormed a Shia mosque, and opened fire, killing one and injuring three. Again, Bangladesh officials doubted IS claims of responsibility and alleged that the group's leaders were simply seeking media attention.

In December, IS claimed responsibility for a suicide bombing at a mosque in the Rajshahi region as about 100 minority Ahmadiyya Muslims gathered for Friday prayers, injuring ten.

But Bangladesh is no stranger to violence driven by religious fundamentalism. Hefazat-e Islam is a coalition of extremist factions who came together to challenge Bangladesh's traditional secularism and politics. They first rose to notoriety after they murdered the young blogger Rajib Haider. Ansar Bangla is another extremist group active in Bangladesh who have frequently claimed responsibility for killing bloggers or secular academics for their alleged advocation of atheism.