Independent tests show that Argentine special prosecutor Alberto Nisman was murdered, his family says.
At a news conference, Mr Nisman's ex-wife said the findings ruled out theories of accident or suicide.
Mr Nisman was found dead in his home on 18 January, hours before he was due to testify in Congress against President Cristina Fernandez de Kirchner.
He was probing Argentina's deadliest terrorist attack, the 1994 bombing of a Buenos Aires Jewish centre.
Sandra Arroyo Salgado, Mr Nisman's ex-wife, said a team of experts had made their conclusions based on reports from a post-mortem examination and existing forensic evidence.
Ms Arroyo Salgado, a federal judge, added that her report was at "the disposal of the prosecutor and judge" involved with the official investigation into Mr Nisman's death.
Despite telling journalists the report's conclusions, Ms Arroyo Salgado has not released the full details of the document.
What the family report concludes:
- There were no spasms in Mr Nisman's right hand, suggesting he had not shot himself
- Additionally, no gunpowder residue was found on his hands
- The prosecutor had been shot in the back of the head
- Mr Nisman's body had been moved to the bathroom once he was shot
- Claims that the prosecutor was drunk are false.
After presenting the report, Ms Arroyo Salgado expressed her disappointment that family members at the scene were not allowed to participate in Mr Nisman's autopsy, despite having the right to do so.
However, she said Mr Nisman's family had decided against conducting a second autopsy.
Following the press conference Viviana Fein, who is leading the investigation into Mr Nisman's death, told Argentina's Radio Nacional that she would consider Ms Arroyo Salgado's report.
Ms Fein told the station: "Up until now ... there has been nothing which allows me to say categorically whether this was a suicide or homicide. Nothing."
One of the experts who helped compile the report was Daniel Salcedo, a former Buenos Aires police chief.
He was assisted by a forensic expert and forensic pathologist.
Analysis: Ignacio de los Reyes, BBC News, Buenos Aires
These new revelations are likely to add even more pressure to the investigation of the most controversial case in Argentina's recent history. They thrust the prosecutor in charge of the investigation, Viviana Fein, into the spotlight.
For weeks she has faced criticism for refusing to say if Alberto Nisman was killed or took his own life, only describing the case as a mysterious death.
But for Mr Nisman's ex-wife there is only one mystery to be solved: who killed him? From the beginning of the investigation, Sandra Arroyo Salgado distrusted the way the first tests were carried out and appointed her own team of analysts.
Among other shocking revelations in her report, she says Mr Nisman died in agony and alleges that his body was moved after he died.
The nation's eyes are now on the Argentine justice system, which is facing some of its toughest tests ever.
It is expected to solve the death of the man who created a political earthquake, and to decide whether to investigate the accusations made against the president and some of her highest ranking officials.
Ms Arroyo Salgado's announcement comes a day after an Argentine prosecutor appealed against a judge's decision last week to throw out Mr Nisman's case against President Fernandez.
In a 350-page report, Mr Nisman had suggested Ms Fernandez covered up the alleged involvement of senior Iranian officials in the 1994 bombing of the Jewish centre.
Ms Fernandez has always rejected the allegations, claiming Mr Nisman was fed misleading information by a rogue intelligence agent trying to discredit her government.
Eighty-five people died in the car bomb attack, which completely destroyed the seven-storey Argentine Israelite Mutual Association (Amia) cultural centre in the capital Buenos Aires.