An Argentine prosecutor is formally declaring for the first time that the man probing the country's biggest terror attack was murdered rather than possibly committing suicide.

A member of the prosecution team said Wednesday it's up to a judge whether to accept the opinion by Eduardo Taiano, who is also seeking testimony by a former aide to the slain Alberto Nisman. The source spoke on condition of anonymity due to the agency's rules.

Computer specialist Diego Lagomarsino owned the gun that killed the crusading prosecutor in 2015. The aide has said he'd loaned the gun to Nisman for protection.

Nisman's death came four days after he formally accused then-President Cristina Fernandez of covering up Iranian officials' role in a 1994 bombing that killed 85 people at a Jewish community center.