Ananta Bijoy Da

Earlier this month, blogger Ananta Bijoy Das was killed in Bangladesh, where four people attacked him with machetes on a crowded street in the city of Sylhet.

The murder was presumably a response to Das’s writing, which “focused on science and evolution” and “criticized some aspects of Islam and also of Hinduism,” according to The Guardian. Das was a contributor to Mukto-Mona (“an Internet congregation of freethinkers, rationalists, skeptics, atheists & humanists”), where he won an award for his “deep and courageous interest in spreading secular and humanist ideals and messages” in 2006.

aMukto-Mona’s founder, Avijit Roy, was himself  “hacked to death” in Bangladesh in February, and Washiqur Rahman, another blogger known for his atheist views, was killed the following month, in March.

The three murders this year bring the total number of writers who have been attacked in Bangladesh since 2013 to six, and have recently prompted more than 150 authors from around the world to sign a letter calling on the country’s government “to ensure that the tragic events of the last three months are not repeated, and to bring the perpetrators to justice.”

The letter is a joint initiative from PEN International and English PEN and carries signatures from Margaret Atwood, Salman Rushdie, Yann Martel, Karl Ove Knausgaard, Amitav Ghosh, and Colm Tóibín, among others.

Jo Glanville, Director of English PEN, said:

This is a campaign of violence against bloggers and writers who are courageous enough to speak out in a hostile culture for free speech. The government of Bangladesh must urgently address the climate of impunity and be seen to safeguard freedom of expression. These shocking events have united writers throughout the world in an important show of solidarity.

TIME reports that since the letter was released, authorities in Bangladesh have banned Ansarullah Bangla Team, a radical Islamist organization suspected of involvement in the murders.