Sarabjit Singh could not have been fatally attacked within the prison "without the knowledge and support of prison guards and the authorities", Pakistan's leading rights watchdog said Thursday.
The Human Rights Commission of Pakistan demanded action against "all those who played any part in the assault" on Sarabjit, who died in Lahore's Jinnah Hospital after being comatose since the brutal assault on April 26.
"Not even the most naive person can believe that a prisoner like Sarabjit, in a death cell inside a jail, can be targeted in such a brutal assault by prisoners without the knowledge and support of prison guards and the authorities," HRCP said in a statement.
"It was no secret that Sarabjit faced more threats than other prisoners on account of the charge that he was convicted of and yet his security was so completely compromised," the statement said.
Sarabjit, 49, was convicted of alleged involvement in a string of bomb attacks across Punjab that killed 14 people in 1990.
His lawyer Awais Sheikh had alerted authorities of threats to his client following the hanging in Delhi of Afzal Guru over the 2001 terror attack on Indian Parliament.
HRCP said the attack on Sarabjit was "far more serious a crime than allowing someone like Gen Pervez Musharraf to escape from court".
The group was referring to Musharraf's escape from the Islamabad High Court after a judge recently ordered his arrest over the imposition of emergency in 2007.
Pakistan and India should take urgent measures to prevent the murder of Sarabjit "from undermining bilateral ties and to improve the lot of detainees from the other country in each other's prisons", the organisation said.