BUENOS AIRES (Reuters) - A prosecutor who accused Argentina's president of orchestrating a cover-up in the investigation of Iran over the 1994 bombing of a Jewish community center was found dead in his apartment with a gunshot wound to the head, the government said on Monday.
The body of Alberto Nisman, who for a decade investigated the blast at the Buenos Aires AMIA Jewish community center that killed 85 people, was discovered on Sunday night.
He had been due to testify on Monday at a congressional inquiry into his allegations.
Preliminary autopsy results suggested "there was no third-person intervention in Nisman's death," the office of state prosecutor Viviana Fein said in a statement. But she also said she "could not rule out a provoked suicide" whereby someone forced or blackmailed Nisman to kill himself.
A 22-calibre handgun and a single bullet casing were found next to his body, the security ministry said.
Senior politicians said they suspected more than a straightforward suicide.
"We want to know which mafiosi sector pushed the prosecutor to take this decision," said Julian Dominguez, who leads the ruling coalition in the lower house of Congress.
Nisman had a large security detail due to threats but seemed combative rather than frightened in recent interviews.
He alleged last week that President Cristina Fernandez opened secretive discussions with Iran and at least one of the men suspected in the bombing and that the scheme aimed to clear the suspects so Argentina could swap grains for much-needed oil from Iran, which denies any connection with the attack.
Tens of thousands of Argentines took the streets on Monday evening to protest Nisman's death and call for justice.
At least 2,000 demonstrated outside the presidential residence in Buenos Aires, chanting "murderer" and hitting their umbrellas against police barricades.
Fernandez published a long, rambling note on Facebook late on Monday, exhorting Argentines to consider the full complexity of the case and not believe the "lies".
"There is not just astonishment and question marks, but also a history that is too long, too heavy, too difficult, and above all, very sordid," she said.
In an apparent move to show she had nothing to hide, Fernandez asked for intelligence information that had been requested by Nisman to be declassified.
The case could become a major issue ahead of October's presidential election. Opposition politicians such as frontrunner Sergio Massa quickly called for a transparent inquiry into Nisman's death and the AMIA bombing.
"This death, which is an inflection point in the history of Argentina's democracy, should serve Argentine society in finding the path of truth regarding the 1994 AMIA bombing," he was quoted as saying by local media.
Nisman's security guards alerted his mother on Sunday afternoon that he was not answering his phone or the front door of his apartment in a high-rise block in the luxurious Puerto Madero district of Buenos Aires.
She found the door to his apartment locked from the inside and got a locksmith to open it. She found her son's body on the floor of the bathroom.
"He was alone in the apartment, there are no witnesses," prosecutor Fein said.
The Clarin daily newspaper reported that Nisman said in an interview just a few days earlier that "I could end up dead because of this." Israel issued a statement mourning Nisman's death and urging Argentine authorities to carry on his work. Argentina's main Jewish organizations, AMIA and DAIA, praised his "inalterable impulse to get to the truth."
But the judge handling the case of the 1994 bombing criticized Nisman late last week for taking it upon himself to "initiate an investigation without judicial control" and said the evidence he put forth was flawed.
Argentine courts have accused Iran of sponsoring the 1994 bombing, a charge Tehran denies. In 2007, Argentine authorities secured Interpol arrest warrants for five Iranians and a Lebanese over the bombing.
In 2013, Fernandez tried to form a "truth commission" with Iran to investigate jointly. She said it would reactivate the inquiry but Israel and Jewish groups said the move threatened to derail criminal prosecution of the case.
The truth commission pact was struck down by an Argentine court and never ratified by Iran.
Nisman had said the commission was intended to help get the arrest warrants dropped as a step toward normalizing bilateral relations and opening the door to obtaining Iranian oil needed to help close Argentina's $7 billion per year energy deficit.
Additional reporting by Walter Bianchi, Enrique Marcarian, Juan Bustamente and Hugh Bronstein in Buenos Aires and Dan Williams in Jerusalem; Editing by Cynthia Osterman and Kieran Murray