The following is a script from "Who Killed the Prosecutor?" which aired on March 8, 2015. Lesley Stahl is the correspondent. Shachar Bar-On, producer.
In the late hours of a January night in Buenos Aires, a dashing, driven Argentine prosecutor was found dead in his apartment. Bullet to the head. It was the day before he was to publicly present his evidence that the country's president, Cristina Kirchner, had secretly conspired with Iran to cover up Iran's involvement in Argentina's worst terrorist attack 20 years ago.
It immediately became an international whodunnit, in great part because of the key players: a hard-charging, high-profile prosecutor and a fiery, populist president, with a penchant for high-drama. What everyone wants to know is: was it murder? Was the government somehow involved? Or was it suicide?
Forensic teams scoured the crime scene for clues: only the victim's DNA was found. His name was Alberto Nisman, was 51, a father of two. He was a prosecutor with 10 bodyguards and a long list of enemies. In the days before his death, he had grown worried for his and his family's safety. Just hours before he died, he asked Diego Lagomarsino, a tech worker in his office, to loan him his gun.
Lesley Stahl: So as far as we know, you were the last person who saw Nisman one day before he died. Did he say why he wanted the weapon?
Diego Lagomarsino: [translated from Spanish] He told me, "Do you know how it feels that your daughters don't want to be with you because they are afraid that something will happen to them by being next to you?" I had never seen Nisman so concerned.
Lesley Stahl: Didn't you say to him, "Why do you need a gun? You have 10 body guards?"
Diego Lagomarsino: Sí. [translated from Spanish] That's the first thing I asked him. And he answered: "Because I don't trust my bodyguards."
Sunday morning, January 18, Nisman is up in his apartment, here up on the 13th floor, but things aren't quite right. He doesn't respond to repeated phone calls from his body guards. We know from the coroner that he died around 3 p.m., in his bathroom, his body was slumped against the door. He had been struck by a bullet at point-blank range above the right ear.
"He told me, 'Do you know how it feels that your daughters don't want to be with you because they are afraid that something will happen to them by being next to you?' I had never seen Nisman so concerned."
Lesley Stahl: Did it enter your mind at all ever in being with him that he might use this weapon on himself?
Diego Lagomarsino: No.
Lesley Stahl: No sense that he was going to kill himself?
Diego Lagomarsino: No.
Lesley Stahl: Do you think he did commit suicide?
Diego Lagomarsino: I don't know.
It looked a suicide, but there was no note and no gunpowder residue on Alberto Nisman's hands. And a forensics team hired by his ex-wife ruled it a murder.
It's a mystery with roots going back 20 years to this crime: the suicide bombing of AMIA, the Buenos Aires Jewish community center that killed 85 -- burying them under the rubble. It was the worst terror attack in this country's history. It's long believed that Hezbollah operatives carried it out; but who sent them? After 10 years with no answer, Alberto Nisman was named special prosecutor in 2004 to find out.
Gustavo Perednik: He believed in it as a mission. He always thought that through justice you can defeat terrorism.
Gustavo Perednik, a close friend who wrote a book about the case, said Nisman, after investigating for two years, charged Iran with ordering and financing the attack.
Lesley Stahl: He went to the very top of Iran and accused the topmost people of approving the bombing.
Gustavo Perednik: And thanks to him they have to be in Iran all the time, because if they leave Iran, then Interpol can look for them.
That's because Interpol issued red alerts, the equivalent of arrest warrants, to several high-ranking Iranians, including the defense minister. Alberto Nisman would spend the rest of his life trying in vain to get them into an Argentine court. So he was stunned when two years ago his country's foreign minister signed a memorandum with Iran in which the two countries agreed to interrogate the accused officials together.
Gustavo Perednik: The memorandum itself is a joke. Because it's meant basically that Argentina has to give to Iran all the results of its investigation against Iran for the Iranians to check. That was absurd.
Prosecutor Nisman felt betrayed, saying this was negotiating with the terrorists responsible for the attack. He saw it as part of the government's tilt towards anti-Western regimes, shepherded by the foreign minister, Hector Timerman, who agreed to talk to us at the foreign ministry.
Lesley Stahl: There's been a lot said about your government shifting policy, foreign policy, away from the United States.
Hector Timerman: Why do you think that an alliance with United States is the only way a country can have a foreign policy? We have an independent foreign policy.
Lesley Stahl: Independent of the United States?
Hector Timerman: Independent foreign policy!
Lesley Stahl: But you used to be very close to the United States. And that is no longer the case.
Hector Timerman: I don't think it is so good to be close to anybody.
Problem is Alberto Nisman was close to the Americans and stayed close. According to leaked State Department cables, Washington was always pressuring him to keep the focus on Iran.
Lesley Stahl: Do you agree with those who say that Mr. Nisman was a puppet of the United States and the CIA?
Hector Timerman: Well, puppet is a very strong word to use by a foreign minister.
Lesley Stahl: Under the sway of?
Hector Timerman: Mr. Nisman used to go to the American embassy here in Argentina and tell in advance what he was going to do.
Lesley Stahl: Do you know if he was about to lose his job? Was he going to be fired?
Hector Timerman: No, he was not going to lose his job.
Lesley Stahl: You've read the speculation.
Hector Timerman: There was speculation in the media but there was no way that we ever discussed taking the job away from Mr. Nisman.
To this day, the case of the bombing of the Jewish community center remains unadjudicated, the evidence gathering dust in a giant warehouse in downtown Buenos Aires: over half a million yellowing documents...folders...audio-cassettes. And now, Argentina has a new mystery because two months ago prosecutor Nisman went on television with an explosive accusation:
[Nisman on TV: Cristina Fernandez De Kirchner...]
That President Cristina Fernandez de Kirchner and her foreign minister had negotiated a secret deal with Iran to improve trade in exchange for dropping those red alerts. Perednik said Nisman saw this as colluding with the enemy.
Gustavo Perednik: This time he said, "I'm going to put them in jail."
Lesley Stahl: I'm gonna put them in jail.
Gustavo Perednik: And "them" is the president and foreign minister.
But four days after his accusation Nisman was dead. President Kirchner - not one to hold back - immediately took to Facebook to suggest it was suicide; then abruptly backtracked and called it murder. People took the streets in protest, calling for justice. Polls show a majority here think he was assassinated...and the president's opponents think she had a hand in it.
This country has a history of assassinations. For years, under military dictatorship, political rivals simply disappeared, but Argentinians thought they had put that behind them.
Lesley Stahl: I've heard people talk about Argentina being drawn back into its dark past.
Hector Timerman: Nah.
Lesley Stahl: Assassinations.
Hector Timerman: Nah, it's impossible. We are talking about things that were terrible, terrible.
Lesley Stahl: One of the great mysteries is whether Mr. Nisman killed himself or whether he was murdered. So what do you think?
Hector Timerman: That I am part of that mystery. I don't know.
He's part of the mystery because he was one of Nisman's targets. The day after he died, the prosecutor was scheduled to appear before Congress to publicly present a 300-page report backing up his charges.
Lesley Stahl: He said that you in 2011 met in Syria with your Iranian counterpart. He said that you said - this is a quote in the report, "Argentina is no longer interested in solving the attack," the bombing here. "It prefers to improve its trade relations with Iran."
Hector Timerman: Well, that's a lie.
Lesley Stahl: Now, that's a direct--
Hector Timerman: That's a total lie. I never said that. Mr. Nisman never showed any evidence that I said that.
Lesley Stahl: He didn't have that on tape or anything like that?
Hector Timerman: --he cannot have something in tape because I didn't say it.
Lesley Stahl: Nisman says that you offered to lift the red alerts.
Hector Timerman: That is-- it's total illogic. I mean, you have to read the law. The only who can cancel the red notice is a judge. I am not allowed.
Lesley Stahl: You could not make a deal to, you could not promise to?
Hector Timerman: I cannot do it.
So is it possible that the prosecutor's charge against the president and foreign minister was without merit? In an evidence room stacked two stories high with investigation AMIA documents, we met the Argentine judge who issued those original red alert arrest warrants.
Lesley Stahl: Now, you were the judge who made the request of Interpol to--
Judge Corral: Yeah. Yes. Yes.
Lesley Stahl: --arrest these people, those red alerts. Yes. Did this government ever ask you to ask Interpol to lift the red alerts?
Judge Corral: No. Never.
Lesley Stahl: Never?
Judge Corral: No government. No person. No nobody.
It seems Interpol never got a request to lift the arrest warrants from anyone. Two days after Nisman made his accusation -- the former head of Interpol sent this letter to the foreign minister affirming that the government was "100% committed that the red alerts remain in effect."
Lesley Stahl: So 180 degrees from what Nisman said.
Hector Timerman: Exactly. And I read that in public.
How could such a seasoned prosecutor make such huge mistake? President Kirchner speculated that her enemies fed Nisman the erroneous information, then killed him hoping she would get blamed. She pointed, for example, to the man who gave him the gun.
Lesley Stahl: So the president named you as someone suspicious in this case. She says that you're an agent of the opposition.
Diego Lagomarsino: No.
Lesley Stahl: Here are some of the rumors we've heard about you: that you're a spy.
Diego Lagomarsino: No [translated from Spanish] I'm not a spy.
Lesley Stahl: That you're an agent of a foreign government. Iran, Israel.
Diego Lagomarsino: No.
We haven't seen any evidence to back up the president's charges about Lagomarsino. A more credible target is this man: Jaime Stuisso, a powerful and shadowy spymaster, who was Nisman's main source for years. One theory is that Stuisso concocted the accusations against the president in revenge after she fired him.
Hector Timerman: Stuisso was fired December, end of December.
Lesley Stahl: Right.
Hector Timerman: And 14 days later, Mr. Nisman accused the president and myself.
Lesley Stahl: Ah, you see a connection there?
Hector Timerman: Oh yes.
The foreign minister suggests that the spymaster manipulated Nisman into coming out with the sensational indictment against the Argentine government before the details could be verified. Those who believe it was suicide think the prosecutor came to realize he had been duped - and snapped.
Lesley Stahl: Is it possible in your mind that he thought to himself, "Oh, my. I got this terribly wrong. And now I don't wanna be embarrassed and have to testify publicly"? Does that-- is that in your head?
Hector Timerman: I cannot imagine what can go through the mind of a person like Nisman. And I don't want to speculate because the man is dead. He cannot defend himself, and so I will not speculate on what happened with him.
There's a new building and memorial on the site of the 1994 AMIA bombing, a crime the prosecutor dedicated 10 years of his life to. Now that he's gone, the families of the victims fear they will never have justice. At the Jewish cemetery in Buenos Aires, Luis Czyzewski visits the grave of his daughter Paola, often. Just steps away is a fresh grave: that of Alberto Nisman.