BUENOS AIRES — Police sentries guarded the federal prosecutor’s luxury high-rise building. His door on the 13th floor had been locked from the inside, and a gun with a spent cartridge was found on the floor near his body. There was no suicide note.

Just one day earlier, on Saturday, the prosecutor, Alberto Nisman, said, “I might get out of this dead.”

From the moment 10 years ago when he was assigned to investigate the 1994 suicide bombing of a Jewish center here that left 85 people dead, Mr. Nisman, an even-keeled lawyer, became entangled in a labyrinthine plot that he traced to Iran and its militant Lebanese ally, Hezbollah.

But it was only in the past week that Mr. Nisman, 51, leveled explosive accusations that top Argentine officials, including President Cristina Fernández de Kirchner, had conspired with Iran to cover up responsibility for the bombing as part of a deal that would supply Iranian oil to Argentina. Now, the mystery has deepened with the discovery of Mr. Nisman’s body on Sunday — the day before he was to testify before lawmakers about those accusations.