An Argentine prosecutor investigating the death last year of Alberto Nisman, four days after he accused the country's then President Cristina Fernández of covering up for Iran in the investigation into the bombing of the AMIA Jewish center in 1994, has said he is sure it was murder.

"There is no doubt that it was not Alberto Nisman who fired the gun that killed him. This leads to the conclusion that it was murder," Ricardo Sáenz wrote to the Buenos Aires Criminal Appeals Court. It marked the first time that an official described Nisman's death as a homicide.

Argentina's new president, Mauricio Macri, has pledged to get to the truth of what happened.

Nisman who was found dead lying in a pool of blood in his bathroom with a shot wound at the head on 18 January 2015. He had been due to go before Argentina's Congress to explain his accusations against President Fernández, and other high government officials, that they conspired to derail the investigation into the 18 July 1994 terror attack in Buenos Aires, which killed 85 people and wounded hundreds, and to cover up the involvement of seven Iranians implicated in the bombing in exchange for favorable trade deals between Argentina and Iran.

Nisman was appointed special prosecutor for the AMIA bombing case in 2004 by Cristina Fernández' predecessor as Argentine president, her husband Néstor Kirchner.