That seems fitting whether his death over the weekend turns out to be a suicide, as the government of President Cristina Fernández de Kirchner seems eager to establish, or the murder of a man who had learned too much.

This much is clear: The best hope of definitively establishing the truth about the horrific July 18, 1994, bombing and its tortured, politicized investigation is to impanel an international team of jurists who can take a fresh, objective look at the evidence.

The truth became more important after Mr. Nisman’s body was discovered inside his apartment on Sunday, hours before he was expected to testify on his startling allegation that Ms. Kirchner cut a deal with the Iranian government to protect some of the culprits of the attack.

Ms. Kirchner’s late husband, former president Néstor Kirchner, appointed Mr. Nisman to lead the inquiry a decade ago after acknowledging a shocking series of blunders by those assigned to the case over the years. Mr. Nisman, 51, became convinced that the bombing was carried out by the Shiite militant movement Hezbollah under orders of the Iranian government. Based on the prosecutor’s investigation, Argentina got Interpol to issue arrest notices for seven prominent Iranians, including a Hezbollah leader and several government officials.

In a strange twist, in January 2013, Argentina announced it had reached a deal with Iran to establish a “truth commission” that would investigate the case jointly. The notion that Iran could be counted on to play a constructive role was questionable at best and rightly met with skepticism from Argentina’s Jewish community.

Earlier this month, Mr. Nisman unveiled a 300-page complaint charging that Ms. Kirchner and Foreign Minister Héctor Timerman conspired to absolve top Iranian officials in exchange for commercial deals that would benefit both cash-strapped governments. President Kirchner issued stern denials. Mr. Nisman said he would corroborate the allegations in a congressional hearing on Monday.

According to the Argentine press, there were no signs that Mr. Nisman was suicidal in recent days. Over the weekend, he prepared for the hearing. He wrote a grocery list for his maid to run to the store on Monday. Despite having a 10-person security detail, Mr. Nisman signaled that he was concerned for his safety, telling an Argentine journalist, Natasha Niebieskikwiat, on Saturday: “I might turn up dead from all this.”

Ms. Kirchner, a pugnacious stateswoman who has been vindictive toward enemies in the press and politics, weighed in on the case in a lengthy, rambling statement posted on her personal website late Monday. “Suicide,” she wrote, is always a befuddling act. “What led a person to make such a terrible decision and end their life?” Missing from her note was a message of condolence for Mr. Nisman’s family. But the most glaring omission is the fact that the manner of death is far from a settled question.