The Argentine prosecutor who died mysteriously while pursuing a case against President Cristina Kirchner was "assassinated," his ex-wife has said, after commissioning an unofficial probe.

Alberto Nisman, who accused Kirchner of shielding Iranian officials from prosecution over the 1994 bombing of a Buenos Aires Jewish centre, "was killed, his death is an assassination," Sandra Arroyo Salgado, a judge who is his ex-wife and mother of his two daughters, said on Thursday.

According to result of the private probe, Nisman did not reportedly die instantly, and that "he had agonised and suffered to some degree," Salgado was quoted as saying. 

The family commissioned its own independent investigation into his death, initially labelled a suicide.

An official inquiry is ongoing.

While the result of the private investigation is unofficial, Al Jazeera's Lucia Newman, who is reporting from neighbouring country Chile, said it was an "important development."

"This seems to have confirmed what everybody believed, but nobody could prove," Newman said. "The death of the prosecution still leaves many, many questions unanswered. 

Nisman was found dead in his bathroom on January 18, on the eve of congressional hearings where he was due to present his accusations against Kirchner.

The case has spawned conspiracy theories involving rogue intelligence agents and a group of Iranians accused of links to the Buenos Aires bombing 21 years ago, or the Israel spy agency Mossad.

Messages from the government have been contradictory, leaving Argentines scratching their heads over the prosecutor's death.

Source: Al Jazeera and agencies