(CNN) An Argentine prosecutor who accused top government officials of a cover-up in the country's deadliest terror attack was murdered, a federal judge said Tuesday.

The ruling marks the first time a judge has called the death of public prosecutor Alberto Nisman a murder, in a case that has been rife with speculation and conspiracy theories.

Nisman was found dead in his Buenos Aires apartment on January 18, 2015, from a gunshot wound to his head. Days earlier, he had filed a report accusing former President Cristina Fernández de Kirchner and other officials of covering up Iran's involvement in a 1994 Jewish community center bombing that left 85 people dead and more than 300 wounded.

Initially, Fernández and others called his death a suicide. But tests found no gunpowder residue on Nisman's hands or traces of blood on his arms to suggest the bullet wound was self-inflicted, judge Julián Ercolini wrote in a 656-page ruling.

"The death of Prosecutor Nisman was not a suicide, and was brought about by a third party and in a painful manner," Ercolini said in the ruling.

Read More