Government officials called it a suicide, but theories of something more sinister arose immediately.
Protesters took to the streets Monday night near the presidential palace -- the Casa Rosada -- waving Argentine flags and holding signs proclaiming, "Yo soy Nisman," or "I am Nisman."
In a report Nisman filed last week, he alleged that Argentine President Cristina Fernandez de Kircher and Foreign Minister Hector Timerman, among others, covered up Iran's involvement in the bombing of a Jewish community center more than 20 years ago that left 85 people dead.
The Fernandez government pushed the suicide theory, pointing to the evidence: Nisman was found dead from a gunshot wound to his temple in his apartment bathroom, a .22-caliber gun and a shell casing near his body.
But a test for gunpowder residue on Nisman's hands came back negative, said Viviana Fein, the federal prosecutor leading the investigation into his death.
Fein said the result does not rule out the possibility that Nisman shot himself -- it only shows that no gunpowder residue was found, as would be expected if he pulled the trigger.
Fein had said earlier that the medical examiner concluded that there was no "third party" involved in Nisman's death. An investigation into the suicide claim has to include a look at whether such an act was provoked or "induced" by some other person, she said.
For protesters in Buenos Aires' Plaza de Mayo, the prosecutor's death struck a deeper chord.
"The killing of Nisman will mobilize the Argentine people to put an end to the corruption, to end this state of disaster that we live in here in Argentina," one protester said.
One woman compared the country's politicians to the mafia, and another said that Argentine democracy was not a democracy anymore.
The investigation into the deadliest bombing in Argentina's history led Nisman down a trail that he seemingly understood to be dangerous.
After filing the complaint last week against the President, foreign minister and other officials, he made a prescient comment about the investigation.
"I could end up dead from this," Nisman told a reporter
His premonition came true the next day.
Diego Guelar, a former Argentine ambassador to the United States, told CNN en Español that the suicide explanation is "ridiculous."
Nisman was committed to his investigation, which he had been overseeing since 2004, Guelar said.
The prosecutor voluntarily wanted to testify before Congress about what he considered irrefutable evidence of a governmental cover-up regarding the 1994 bombing, Guelar said.
"Without a doubt, this is an incident that has shocked all Argentines very strongly," he said.
The Fernandez administration hit back sharply after Nisman's complaint was filed last week, denying that there was a cover-up about Iranian links to the bombing of the Argentine Israelite Mutual Association building.
Prosecutors in Argentina have alleged for years that Iran was behind the attack, but a criminal investigation has stalled, in part because Iran denies the charges and has not made the accused available to prosecutors.
Argentina requested the arrest of several Iranians in 2007 in connection with the bombing, including the current defense minister of Iran, Ahmad Vahidi. Interpol approved so-called Red Notices for these suspects, alerting countries around the world of Buenos Aires' desire to have them arrested.
To surpass this roadblock, Fernandez signed an agreement with Iran to create a joint commission to investigate the bombing. She said it would move the investigation along, while critics said such a probe would only protect the Iranian suspects.
In his report, Nisman alleged that Fernandez and Timerman wanted to cover up Iran's involvement in order to boost trade -- specifically, oil imports and grain exports -- with the Islamic republic.
"Suicide provokes, in all cases, first: disbelief, and then: questions. What was it that led a person to make the terrible decision to take his own life?" Fernandez wrote in a lengthy letter she posted on her Facebook page.
She hailed her own involvement in the investigation into the bombing, dating back to her time as a lawmaker in the early 2000s. As the investigation progressed, politically-motivated conspiracy theories and allegations arose, she said.
"As time goes on, instead of clarifying, everything becomes more opaque," Fernandez wrote. "It became complicated and there was a growing sensation, at least for me, of being in a theater of national and international politics, in which the victims were important to very few, and the truth even more so."
Nisman's allegations of a cover-up, she said, "were intended to lie, disguise and confuse."
In the wake of Nisman's death, a judge has ordered that all of the the evidence collected by the prosecutor be preserved, said Patricia Bullrich, an Argentine lawmaker and head of the committee that invited Nisman to testify.
Nisman's role was to investigate the bombing, in which eight Iranians had been charged, Bullrich told CNN en Español.
In the course of that investigation, Nisman came upon evidence that led to his claims of a cover-up, the lawmaker said.
After Nisman filed his complaint with the new allegations last week, it was Bullrich who invited him to speak before Congress. He was found dead just hours before he was supposed to testify.
"In the days before his death on Sunday, the prosecutor was very active, very focused on the presentation he was going to give before Congress, in the evidence he was going to present and his mind made up about going forward with it," Bullrich said. "It's hard to believe that he would have taken his own life."