An Iranian pathologist found that the son of a prominent conservative politician's adviser died from beatings and poor prison conditions, it was reported.
The report, by the semi-official Mehr news agency, could not be independently confirmed, but if true, would be the first official confirmation of a prisoner abuse death during Iran's post-election turmoil.
It could be a sign that conservatives will seek to prosecute those responsible for prisoner abuse in an attempt to defuse anger over allegations that many opposition supporters detained after the disputed presidential election were tortured.
The claims have outraged many conservatives, as well as the pro-reform opposition that believes hardline President Mahmoud Ahmadinejad stole the June election through massive vote fraud.
Conservative anger has partly been driven by the death of Mohsen Rouhalamini, the son of an adviser to defeated presidential candidate Mohsen Rezaei.
Police initially suggested that Mr Rouhalamini's death in custody was caused by meningitis. But Mehr, which has close ties to conservatives, said yesterday that a state forensic doctor ruled that out in a report handed over to judicial authorities about 10 days ago.
Instead, it said Mr Rouhalamini died of "physical stress, the effects of being held in bad conditions, multiple blows and severe injuries to the body", according to the news agency, which cited "informed sources".
Conservative critics of Mr Ahmadinejad have in the past used the news agency to leak information.
Iran's supreme leader Ayatollah Ali Khamenei recently appointed the brother of Mr Ahmadinejad's main conservative rival to head the judiciary, a move some analysts interpreted as a way to appease conservatives upset over Khamenei's swift support of the president in the election crisis.
The new judiciary chief, Sadeq Larijani, whose brother is parliament speaker Ali Larijani, recently appointed a three-member team to supervise an investigation into the post-election unrest, including the alleged abuse of detained protesters.
The prosecution of officials accused of torture would be an embarrassment for Mr Ahmadinejad, who has staunchly defended the security forces sent out after the June 12 election to violently crack down on the thousands of protesters who took to the streets.
The government has confirmed that at least 30 people were killed during the unrest, but the opposition believes at least 69 died. Hundreds of protesters and opposition activists were also arrested.
Mr Rouhalamini was detained during a July 9 protest and taken to hospital two weeks later where he died within hours. Reformist websites said he had been held at Kahrizak prison, where much of the alleged prisoner abuse took place, and his jaw was broken when his father received his body.
The pathologist's report carried by Mehr said Mr Rouhalamini was "in bad physical condition" before a scheduled move to Evin prison, which is also located in Tehran.
"After his condition was seen to worsen ... and after a 70-minute delay, he was taken to a hospital, where unfortunately he died," said the report.