In a case that has sparked international outrage, 35-year-old Sattar Beheshti who wrote a blog critical of the government was arrested on October 30 after receiving death threats and died some days later, having complained of being tortured.
Under increasing pressure at home and abroad for an investigation, Iran's parliament said it had formed a committee to examine the case and the judiciary said it would deal "quickly and decisively" with those responsible.
"In its latest report, the seven-member medical committee says ... it is not possible to determine the exact cause of death," IRNA quoted Tehran prosecutor's office as saying in a statement.
"But the most likely cause leading to death may be shock," the statement said, adding that excessive psychological stress could have caused the shock.
Iranian authorities arrested seven people suspected of involvement in the death of Beheshti and a judiciary official said a forensic examination had found bruises on five parts of the blogger's body.
The statement on Thursday said investigation was still underway and the results would be made public.
The Beheshti case has renewed international concerns about human rights in Iran. In October, the European Parliament awarded its human rights prize to Nasrin Sotoudeh, a lawyer who, opposition websites report, is on hunger strike in prison.
(Editing by Jon Hemming)