The death of Mahsa Amini, which sparked weeks of protests across Iran, was the result of multiple organ failure caused by cerebral hypoxia, an Iranian coroner's report said on Friday.
Amini, 22, fell into a coma after being arrested by morality police in Tehran and her death was announced on September 16. Her family and activists allege she died as a result of being beaten.
Iran's Forensic Organisation said her death "was not caused by blows to the head and vital organs and limbs of the body", in its report revealed on state television.
The report said Amini fell while in custody due to "underlying diseases".
"Due to the ineffective cardio-respiratory resuscitation in the first critical minutes, she suffered severe hypoxia and as a result, brain damage," it said.
The report said Amini's death was linked to "surgery for a brain tumour at the age of 8".
The findings included CT scans of the brain and lungs, an autopsy and pathological tests.
The lawyer for Amini's family, Saleh Nikbakht, previously told the semi-official Etemadonline news website that "respectable doctors" believe she was hit in custody.
The police have denied she suffered any harm. They previously said she had suffered a heart attack after being taken to a station to be "educated".
Her family deny that Amini had any heart problems.
Since her death, dozens have been killed, mostly protesters but also members of the security forces and hundreds arrested.
Iranian supreme leader Ayatollah Ali Khamenei on Monday accused the United States and Israel of fomenting the wave of nationwide unrest.
"The death of the young woman broke our hearts," said Mr Khamenei.
"But what is not normal is that some people, without proof or an investigation, have made the streets dangerous, burnt the Quran, removed hijabs from veiled women and set fire to mosques and cars."
The 83-year-old leader stressed, however, that police must "stand up to criminals" and said "whoever attacks the police leaves the people defenceless against criminals, thugs, thieves".