Iranian authorities have briefly detained the parents of a dissident student leader, Akbar Mohammadi.
Mr and Mrs Mohammadi were held after they left the aeroplane
The dissident was recently found dead in prison following a hunger strike.
A lawyer for the family, Nemat Ahmadi, told the BBC that Mr and Mrs Mohammadi were held at Tehran airport after they arrived from a trip abroad.
Akbar Mohammadi was serving a 15-year prison term for taking part in a pro-democracy student rally in 1999.
Mr and Mrs Mohammadi were visiting their daughter in Turkey when they received the news of their son's death in Tehran's Evin prison.
They arrived in Iran at midnight where a crowd of friends and family members were waiting to greet them.
Agents from Iran's Intelligence Ministry forcibly escorted them away as soon as they left the aeroplane, and obliged them to travel on to Amol, their home town.
The student leader's father had been particularly critical of the Iranian government, saying his son was tortured while in custody.
Nemat Ahmadi told the BBC the authorities had detained the parents in order to prevent them publicising the circumstances of their son's death.
Another lawyer for the family, Khalil Bahramian, has said the student leader's death is suspicious and he has called for an independent post-mortem examination.
Iranian human rights activists have also demanded that an independent international team should investigate the cause of death.
Akbar Mohammadi was among tens of thousands of students who took part in the pro-democracy demonstration seven years ago.
At the time of his death he was on a hunger strike to protest his prison term.
His brother, Manouchehr, is also in prison for the same reason.
Mr Mohammadi's death in custody comes at a time when Iran is under increasing international pressure because of its controversial nuclear activities and its support for Hezbollah in Lebanon.