The son of an Iranian doctor who was killed after examining the rape victims of the country's 2009 unrest has spoken for the first time about the motives behind his father's assassination.

Abdolreza Soudbakhsh, a physician and professor at Tehran University, was shot dead by men on a motorcycle as he left his office last September. At the time of his assassination, Iranian officials denied his murder had anything to do with the cases of alleged rape in Kahrizak, a detention centre that Iran used to imprison many of the opposition activists caught up in the protests following the country's disputed presidential elections.

Many protesters are believed to have been tortured to death in Kahrizak and several have claimed they were raped. But the doctor's son Behrang Soudbakhsh said in an interview with Fereshteh Ghazi of Roozonline, an opposition website, that his father had indeed examined the rape victims of Kahrizak and was under pressure to remain silent about those who died under torture.

Kahrizak became a scandal for the regime when Mohsen Rouholamini, the son of a former senior advisor to the Revolutionary Guards, was named among prisoners who died in the centre.

After Rouholamini's death, Iran's supreme leader, Ayatollah Ali Khamenei, ordered the closure of Kahrizak but the opposition leader Mehdi Karroubi later spoke out about the extent of rape inside the centre after meeting some of its victims.

"[My father] was told to say that the victims of Kahrizak had Meningitis. He asked to see the dead bodies and when he examined them, he concluded that they had died under torture and not Meningitis," Behrang Soudbakhsh said.

"Once he said that how could they rape an 18-year-old kid so severely that he died after that? How could they rape the children," he asked.

The doctor was planning to leave Iran on the night of his assassination, which his son said suggested he was killed to prevent him from revealing more information. Soudbakhsh Sr had given an interview to Deutsche Welle's Persian network a few weeks before his death, in which he had mentioned rape inside prison.

"They were thinking that my father was going to the US to reveal his information in details in an open society. My father was one of the few experts in Iran who had precise information [on the issue]," Soudbakhsh Jr said. "They killed my father because he didn't want to lie and he didn't lie."

According to the doctor's son, Soudbakhsh was wearing a bulletproof vest at the time of his assassination. He was shot where the vest did not protect his body by a gun with a silencer. His son said this showed the killers were aware he was wearing the vest.

Soudbakhsh Jr said the police refused to co-operate with an inquiry into his father's death. According to witnesses who spoke to the son, but who have spoken in public, the killers appear to have been so unworried about being caught that they did not cover their faces. But the dead man's family has been unable to watch CCTV footage of the incident.

Iran's elite Revolutionary Guards and its intelligence unit are believed to be the main groups responsible for the suppression of protesters in the country's post-election unrest. Families of the dead protesters have appealed to the UN human rights special rapporteur, Ahmed Shaheed, to investigate the events related to the 2009 unrest but Iran has signalled it will not allow the monitor to enter the country.

The scandal has also taken the life of another Iranian doctor, Ramin Pourandarjani, known as the "Kahrizak doctor", who examined the inmates in the detention centre. He died in November 2009 under mysterious circumstances.