(CNN) -

In his writings, author Avijit Roy yearned for reason and humanism guided by science.

He had no place for religious dogma, including from Islam, the main religion of his native Bangladesh.

Extremists resented him for openly and regularly criticizing religion in his blog. They threatened to kill him if he came home from the United States to visit.

On Thursday, someone did.

As usual, Roy defied the threats and departed his home in suburban Atlanta for Dhaka, where he appeared at a speaking engagement about his latest books -- one of them titled "The Virus of Faith." He has written seven books in all.

As he walked back from the book fair, assailants plunged machetes and knives into Roy and his wife, killing him and leaving her bloodied and missing a finger.

Afterward, the Islamist group "Ansar Bangla-7" reportedly tweeted, "Target Down here in Bangladesh."

Investigators are proceeding on the notion that Roy's murder was an extremist attack. His father, Ajay Roy, filed a case of murder with the Shahbagh police Friday without naming suspects.

No one came to their aid as they were hacked down, a witness said. "I shouted for help from the people but nobody came to save him."

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Avijit Roy received death threats prior to visit