Sohel Mahmud, assistant professor in Dhaka Medical College's forensic department, said there were three deep gashes on the right side of Avijit's head caused by a sharp weapon like a machete.

He added the gashes were parallel, only half inches from each other and strongly struck. The blows had cut through the skull and reached the brain causing excessive bleeding.

"This is the handiwork of a professional. They knew where to hit to kill a man," the doctor said adding it was impossible to carry out such an attack without "planning, skill and brutality".

A bio-engineer and a naturalised US citizen, Avijit Roy was hacked to death on the street opposite the Dhaka University Central Mosque after he along with his wife emerged from the Amar Ekushey Book Fair on Thursday night.

His wife and fellow blogger Rafida Ahmed Banna was seriously injured as she tried to defend Avijit. Religious radicals have been threatening Avijit for his active campaign against Islamist extremism.

Avijit's father, Ajay Roy, filed a case with Shahbagh police over the murder. Police are yet to make any breakthrough in the case.

No one has been arrested or detained. A Twitter account in the name of Ansar Bangla 7 has described the murder of Avijit Roy as an achievement. A series of tweets since the murder described it as a punishment for Avijit's "crime against Islam".

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