Iran's hardline judiciary has claimed the death of journalist Zahra Kazemi in custody must have been "an accident".
Kazemi died after falling into a coma
A statement said that was the only option remaining since the "sole defendant" was acquitted last Saturday.
A Canadian embassy official expressed surprise, saying it ran completely counter to everything that had emerged from investigations over the last year.
The case has caused a deep rift within Iranian political circles and outraged human rights organisations.
The Iranian-Canadian journalist died in 2003 after taking photos outside a Tehran jail.
She died in hospital in the capital on 10 July after falling into a coma, having received head injuries during more than three days of interrogation.
The judiciary cleared suspect Mohammad Reza Aghdam Ahmadi, an intelligence ministry agent, of her killing last Saturday "due to lack of sufficient evidence".
"The only suspect in this quasi-intentional murder of Zahra Kazemi was innocent, so there remains only one other option," its statement said.
"This is that the incident leading to the death of the late Kazemi was because of a drop in her blood pressure caused by her hunger strike, thus making her fall from a standing position and get hurt."
The statement lashed out at Iran's reform-minded government, after spokesman Abdollah Ramezanzadeh told reporters on Monday that the intelligence ministry was prepared to identify the alleged real killer if the judiciary allowed it.
"Comments polluted with political intentions will lead only to ambiguity and the disruption and deception of the public mind, as well as provide propaganda material for the spiteful media," the judiciary said.
Ms Kazemi's family has previously said a high-ranking justice department official struck the fatal blow, and that the court was putting someone else on trial to protect the real killer.
Reformist President Mohammad Khatami had also said he believed Mr Ahmadi was innocent and called on the judiciary to identify "the real guilty person".
Zahra Kazemi's son, Stephan Hachemi said the trial was a "cover-up" to divert attention from the real killer.
The case soured relations between Iran and Canada and Ottawa recalled its ambassador in protest.
On Tuesday, Canada said it would ask the European Union and the United Nations to put pressure on Iran to resolve the case.
The judiciary's assertion now that it can only have been an accident is unlikely to be the last word, says the BBC's Jim Muir in Tehran.
He says more likely it will add further fuel to the flames.