Iranian security forces shot dead a 21-year-old man during protests in the city of Amol last month, a source close to his family has told BBC Persian.
Erfan Rezaei was filmed tearing down a government poster showing the supreme leader shortly before he was killed on 21 September, the source said.
He was shot in the shoulder and back by a pistol at close range, they added.
Officials have not commented, but the source said his family was under pressure to say protesters killed him.
Anti-government protests have engulfed Iran since the death in custody six weeks ago of Mahsa Amini, a 22-year-old woman who was detained by the morality police for allegedly wearing her hijab, or headscarf, "improperly".
Iran's Human Rights Activists News Agency (HRANA) estimates that 248 protesters, including 33 children, have been killed by security forces.
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Erfan's mother, Farzaneh Barzekar, was told by officials that he had been admitted to hospital not long after the protest, the source told the BBC.
Nurses at the hospital would not tell her where he was, but after three hours of searching she found his blood-soaked clothes outside an operating theatre and fainted.
The source said Erfan died as a result of severe damage to the kidney and spleen caused by the bullet wound to his back. The bullet was fired by a pistol from a distance of 5m (16ft), they added.
Authorities handed over Erfan's body to his family two days later on the condition that they held a quiet funeral, according to the source.
The source said security forces also pressed them to say that Erfan was a bystander shot dead by "rioters", as Iran's leaders have portrayed the protesters.
The source added that the family was allowed a funeral because Erfan's father was a veteran of the Iran-Iraq War who still suffers from the effects of exposure to chemical weapons and post-traumatic stress disorder (PTSD).
Iranians revere those who fought in the 1980-88 conflict, in which more than one million people were killed.
The families of several other young protesters allegedly killed by security forces during the current unrest have come under similar pressure from authorities.
Last week, sources told BBC Persian that the family of Abolfazl Adinezadeh - a 17-year-old boy who was killed by a shotgun fired at point-blank range at a protest in Mashhad on 8 October - had been told to say he was a member of the Basij, a notorious paramilitary force involved in the crackdown.