There are fears an Iranian man who received death threats due to his anti-government blog was tortured to dead while in police custody.
Amnesty International and European governments have urged Tehran to investigate the death of Sattar Beheshti, 35, who was arrested in his home southwest of the capital on October 28. His body was handed back to his family on Wednesday.
"Fears that Sattar Beheshti died as a result of torture in an Iranian detention facility, after apparently lodging a complaint about torture are very plausible, given Iran's track record when it comes to deaths in custody," Amnesty said.
Beheshti was not well known in a country where media is tightly controlled and blogging has become one of the few outlets - albeit a risky one - for criticising the state.
The day before his arrest he said he had been threatened.
"They sent me a message saying, 'Tell your mother she will soon be wearing black because you don't shut your big mouth'," Amnesty quoted him as saying.
Iran clamped down on dissent after mass protests against the disputed re-election of President Mahmoud Ahmadinejad in 2009. Iranian authorities have acknowledged three people arrested in the post-election unrest were killed in custody.
British Foreign Office minister Alistair Burt said he was shocked by the case.
"Beheshti's only crime appears to be advocating the defence of human rights on the internet," he said in a statement.
"If these reports are true, this is yet another disgraceful attempt by the Iranian Government to crush any form of free expression by its citizens."
France and Britain have called on Tehran to investigate.