Argentina's President Cristina Fernandez has done an about-face on her initial statements that prosecutor Alberto Nisman's death earlier this week was suicide.

Nisman, 51, had been investigating an alleged government cover-up of Iran's suspected role in the 1994 bombing of the Argentine-Israeli Mutual Association community center in Buenos Aires that killed 85 people.

Nisman was found dead on Sunday "slumped in the bathroom of his apartment with a bullet wound in his head. He was lying next to a .22-caliber handgun and a bullet casing," Reuters says. Days earlier, Nisman had accused Fernandez and Foreign Minister Hector Timerman of being involved in the alleged cover-up.

"The suicide (I'm convinced) was not a suicide," Fernandez said in a letter published on social media sites on Thursday.

Reuters notes that Nisman's death "has rocked Argentina, with polls saying a majority of people reject the idea that Nisman killed himself only hours before he was scheduled to detail his allegations before Congress."

Speaking to NPR's Morning Edition, Buenos Aires-based journalist Jonathan Gilbert says Nisman said that eight Iranians masterminded the attack, which was carried out by Lebanon's Hezbollah.

Gilbert says Nisman had accused the government of signing a memorandum with Tehran that was "just a veneer for a secret pact that would exchange the immunity of those Iranian officials [in exchange] for oil imports."

Gilbert says Nisman had been working on the case for a decade and "sent messages to friends just before he was about to talk to the politicians in Congress on Monday, saying essentially that this was the defining moment of his career."

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