The young woman from Kurdistan Province was visiting the capital Tehran with her family when she was arrested for violating the law on covering her hair with a hijab.
Mahsa Amini's brother has said that the morality police - who crack down on women they deem not to cover up enough - slammed her head against their van and beat her with a baton.
Five days later she was dead.
Family: Official cause of death "a lie"
According to the New York Times, authorities have confirmed that she was arrested for violating the hijab law - but without providing more specific details.
- I want to know why she was arrested, why they did this, says Mahsa Amini's mother to Iranian media.
- The grief is killing me.
According to the official information, Mahsa Amini must have died of a heart attack.
A lie, according to the father who chose to give an interview to the BBC's Persian editors.
He is said to have unsuccessfully appealed to the coroner to correct the certificate establishing the cause of death.
Pictures from the deathbed triggered a wave of protest
According to Mahsa Amini's mother, government officials visited the family at the hospital and ordered them not to speak publicly about her daughter's death.
The family chose to release hospital photos of the unconscious Mahsa Amini bleeding from the ears - which could indicate severe head trauma, according to several Iranian doctors who spoke to local media.
The images sparked a massive wave of protests that have now taken place in about sixty Iranian cities, according to the New York Times, and have been pushed back hard by the police.
It is unclear how many have been killed
Media both inside and outside of Iran are struggling to get reliable information as access to the internet has been cut off for periods throughout the country.
According to state-controlled television, there are 26 deceased, including police officers.
But according to the human rights organization Iran Human Rights, at least 50 have been killed, TT reports.
One of them must have been a child, according to information given to Swedish Amnesty International, which writes on Facebook that there is also evidence that "Iranian security forces are using illegal weapons, including shotguns, to disperse the protesters".