"All signs point to suicide," said Argentine Security Secretary Sergio Berni following the death of Alberto Nisman, 51, whose body was found overnight in his apartment in the trendy Puerto Madero neighborhood of the capital.
Nisman, who had accused President Cristina Kirchner of obstructing a probe into a 1994 Jewish center bombing, was found dead of a gunshot wound just hours before he was due to testify at a congressional hearing.
Nisman since 2004 had been investigating the 1994 van bombing of the building of the Argentine Jewish Charities Federation, or AMIA.
The bombing left 85 people dead and 300 others injured in the worst attack of its kind in the South American country.
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Nisman last week had asked for an investigation into possible obstruction by Kirchner and was due to speak at a congressional hearing Monday to provide evidence of his assertions.
Anibal Fernandez, secretary general for the presidency, said he was "dumbfounded" by Nisman's death, saying there was "absolutely nothing normal" about it.
Israel's foreign ministry issued a statement expressing sorrow over Nisman's death, praising him as a courageous jurist who "worked with great determination to expose the attack's perpetrators and dispatchers."
"The State of Israel hopes Argentina's authorities will continue Nisman's work, and take every possible effort to bring those behind the Argentina attacks to justice."
The prosecutor had accused Iran of being behind the attack and said Kirchner hampered the inquiry to curry favor with the Islamic republic and gain access to its oil.
Nisman had also accused former president Carlos Menem (1989-99) of helping obstruct an investigation into the bombing, which has never been solved.
Since 2006, Argentine courts have demanded the extradition of eight Iranians, including former president Akbar Hashemi Rafsanjani, former defense minister Ahmad Vahidi and Mohsen Rabbani, Iran's former cultural attache in Buenos Aires.