The moment American blogger's wife was left covered in blood next to her husband's mutilated body after he was hacked to death by Muslim militants for insulting Islam and 'in retaliation for U.S. strikes on ISIS'

Dr Avijit Roy, 42, from Atlanta, Georgia, was jumped by the gang of machete-wielding assailants Thursday night in Dhaka, the capital of Bangladesh - leaving his injured wife standing over his bleeding body. He is well-known in Bangladesh for for his outspoken atheism.

An obscure militant group, Ansar Bangla 7, claimed responsibility for the attack, which it said was in retaliation for his 'crime against Islam'.

The extremists also said he was singled out because he is a U.S. citizen - and characterized the vicious killing as 'revenge' for attacks on ISIS in Syria. 

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Brutal: This image, circulated by extremists on social media, shows Dr Aijivit Roy dead on the floor in Dhaka, Bangladesh (right) on Thursday night. Left, his wounded wife, Rafida Ahmed, stands over his body

Outspoken: Blogger and author Dr Roy, 42, was described as a 'free thinker' with a 'strong voice against Islamic fanatics,' by a friend

Murder: The body of Avijit Roy was found in a pool of blood after being attacked by a clever wielding gang in Dhaka, Bangladesh on Thursday night

Police said at least two men, who are believed to have links to Dhaka University, attacked Roy around 8.45pm and landed three strong, deliberate blows to the right side of his head.

They turned on his wife, Rafida Ahmed, when she tried to save him, then fled into the crowd, dumping their weapons as they ran. Police have expressed shock that extremists struck at a university book fair, which was heavily guarded.

Witnesses have even said officers and bystanders were there during the murder - but did nothing. Several onlookers told Al-Jazeera nobody intervened in the brazen attack, while even police chiefs had said they were baffled that a man could be murdered so close to the heavily-guarded area.

The United States today said the murder was 'horrific in its brutality and cowardice' and represents an attack on the 'principles' of Bangladesh.

Roy's body was found in a pool of blood on Thursday night after the brutal attack. Ahmed, 45, is also a blogger. She is reportedly now in hospital with a deep cut to her head.

Roy, who is a naturalized U.S. citizen and also a bio-engineer, had been receiving death threats online for years.

One Muslim fanatic, Farabi Shafiur Rahman, had reportedly posted threats on Facebook last year that Roy would be killed as soon as he returned to the country.

Roy and his wife were visiting the city of Dhaka for a book fair when they were set upon opposite the Dhaka University Central Mosque. Police have said the attackers likely have links to the university. 

Jubilant extremists writing on social media were quick to praise the attackers. The account for Ansar Bangla 7 made a post which linked the killing to Roy's nationality - and American airstrikes on ISIS. 

Survivor: Rafida Ahmed is pictured above being stretchered to hospital after the attack on Thursday night. Police say she was attacked after trying to defend her husband

Torches raised: This crowd of women marched through Dhaka Friday night in Bangladesh in protest