The death in custody of a well-known Iranian student activist has brought international condemnation.
Iran is regularly dogged by tales of harsh conditions in its prisons
Akbar Mohammadi was jailed after joining pro-democracy student demonstrations in 1999, and had been on hunger strike to demand his release.
The US condemned what it called Iran's "severe repression" of dissidents.
Authorities say they are investigating the cause of Mr Mohammadi's death earlier this week, and say he was under the care of a doctor in jail.
Amnesty International said his death in custody had cast a pall over the entire Iranian justice system.
The organisation called for an independent investigation and post mortem examination to determine the cause of Mr Mohammadi's death and the conditions that led to it.
US State Department spokesman Sean McCormack said Washington condemned what he called the Iranian government's "severe repression" of dissidents and its continued crackdown on civil society and those fighting for personal freedom.
Without elaborating, he alleged that the regime's actions had resulted in the death of Akbar Mohammadi.
Amnesty International alleged that Mr Mohammadi had been routinely tortured from the day of his arrest in 1999.
It also alleged that the information available strongly indicated that repeated delays or outright denials of adequate medical care by Iran's judicial and prison authorities had contributed to his death in custody.
The organisation also called for an independent investigation into his death.
Amnesty said that Mr Mohammadi had reportedly undertaken a hunger strike, during the last few days of which he had refused liquids as well as solids.
However, Iranian prison officials have been quoted as saying that he was drinking water and tea.
Iran's Justice Minister, Jamal Karimirad, said Mr Mohammadi had been under intense supervision by the prison doctor, but had insisted on going back to his cell.
There his condition again deteriorated and he died on the way to hospital.
The Iranian authorities say the coroner's office is investigating his death. But a number of reformist groups inside Iran have also called for an independent investigation.