The body of Alberto Nisman, 51, who had received threats, was found overnight in his 13th-floor apartment in the upscale Puerto Madero waterfront neighbourhood of the capital Buenos Aires yesterday.
"All signs point to suicide," said Public Safety Secretary Sergio Berni, an assertion backed up by initial forensic findings.
Federal prosecutor Viviana Fein said Nisman died of "a gunshot wound to the temple" and "there was no role of additional parties (in the death)."
Investigators should look at whether Nisman was under pressure from anybody, and who the gun belonged to, local media reports quoted Fein as saying. The weapon was not Nisman's, the reports said.
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Nisman, who had accused Kirchner of obstructing an investigation into a 1994 Jewish center bombing, was due to testify at a congressional hearing yesterday to provide evidence of his claims.
Since 2004 he had been investigating the van bombing of the Argentine Jewish Charities Federation, or AMIA, which left 85 people dead and 300 others wounded in the worst attack of its kind in the South American country.
Anibal Fernandez, secretary general for the presidency, said he was "dumbfounded" by Nisman's death, adding there was "absolutely nothing normal" about it.
Kirchner, in a statement released through her intelligence minister Oscar Parrilli, ordered the declassification of intelligence information Nisman had sought a week ago.
Israel meanwhile expressed sorrow over Nisman's death, praising him as a courageous jurist who "worked with great determination to expose the attack's perpetrators and dispatchers."
Officials said a .22-caliber handgun was found beside Nisman's body, which was discovered by his mother in the bathroom of his apartment after his security detail was unable to contact him.
The prosecutor had accused Iran of being behind the attack and said Kirchner hampered the enquiry to curry favour with the Islamic republic and gain access to its oil.